What's new in the pathology of neuronal cytoskeleton: the significance of neurofibrillary tangles

Transglutaminase activity is increased in Alzheimer's disease brain

Transglutaminase is a calcium-activated enzyme that crosslinks substrate proteins into insoluble, often filamentous aggregates resistant to proteases. Because the neurofibrillary tangles in Alzheimer's disease have similar characteristics, and because tau protein, the major component of these tangles is an excellent substrate of transglutaminase in vitro, transglutaminase activity and levels were measured in control and Alzheimer's disease brain. Frozen prefrontal cortex and cerebellum samples from Alzheimer's disease and control cases matched for age and postmortem interval were used in the analyses. Total transglutaminase activity was significantly higher in the Alzheimer's disease prefrontal cortex compared to control. In addition the levels of tissue transglutaminase, as determined by quantitative immunoblotting, were elevated approximately 3-fold in Alzheimer's disease prefrontal cortex compared to control. To our knowledge, this is the first demonstration that transglutaminase is increased in Alzheimer's disease brain. There were no significant differences in transglutaminase activity or levels in the cerebellum between control and Alzheimer's disease cases. Because the elevation of transglutaminase in the Alzheimer's disease samples occurred in the prefrontal cortex, where neurofibrillary pathology is usually abundant, and not in the cerebellum, which is usually spared in Alzheimer's disease, it can be suggested that transglutaminase could be a contributing factor in neurofibrillary tangle formation.

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.

Mechanisms of cell death in cholinergic basal forebrain neurons in chronic alcoholics

Tau immunoreactivity was examined in post mortem tissue from patients in three groups: neurologically-asymptomatic and neuropathologically normal alcoholics, alcoholics with Wernicke's Encephalopathy (WE) and age matched non-alcoholic controls. Tau-positive granular and fibrillary inclusions were frequently observed within the magnocellular neurons of the cholinergic nucleus basalis, within occasional nucleus basalis neurons in non-WE alcoholics, but not in controls. Tau immunoreactivity was not however observed in cortical, brainstem, diencephalic or non-cholinergic forebrain structures. Peroxidase activity was also examined within the nucleus basalis using diaminobenzidine as an indicator. The majority of neurons in the basal forebrain showed increased peroxidase activity in all WE alcoholics and in some nucleus basalis neurons of non-WE alcoholics, but was rarely seen in controls. Neighboring astrocytes also showed increased peroxidase activity. These results suggest a link between peroxidase activity and the abnormal accumulation of phosphorylated tau. The presence of tau in the nucleus basalis of alcoholics with WE suggests a thiamine-dependent mechanism in tau accumulation and cell death in the cholinergic basal forebrain.