Although older (>65 years) individuals manifest cognitive impairments ranging from mild abnormalities to overt dementia, the molecular and cellular substrates that account for this variation remain to be defined. Several reports indicate cholinergic hypofunction in elderly humans. However, none of these studies related cholinergic changes to cognitive status. The importance of this system to human cognition is underscored by studies in Alzheimer's disease (AD) where the most consistent neurochemical abnormality to crrelate with cognitive impairment is decreases in cortical choline acetyltransferase (ChAT). Interestingly, AD patients with an increased gene dose for the apolipoprotein (ApoE) e4 allele (a genetic risk factor for AD) exhibit greater cholinergic deficts (i.e., cortical ChAT reduction and cholinergic basal forebrain(CBF) neuron loss) and are less responsive to anticholinesterase drugs. The status of the CBF system and the association of the ApoE genotype in aged individuals with mild cognitive impairment (MCI), however, is still unknown. We do know, however, that cortical ChAT levels decrease with aging in the human brain. Therefore, Specific Aim 1 will establish whether CBF degneration including reduction in cortical ChAT is associated with cognitive impairment in individuals with MCI. Findings from our group, have led to the hypothesis that the vulnerability of CBF neurons results from defects in the binding or transport of NGF and its high affinity signal transducing trkA receptor which, in turn, leads to impaired NGF trophic support in AD. Support for this suggestion is derived from our studies demonstrating that NGF protein- immunoreactivity and the message for its high affinity trkA receptors are decreased within CBF neurons whereas, cortical NGF protein is increased in AD. Taken together these observations suggest that the NGF protein accumulates within the cortex, because it is not transported in a normal fashion to CBF consumer perikarya where it is needed for its trophic effects to occur.
Specific Aims 2 -3 will determine whether impaired NGF transport is also associated with age-related MCI in the elderly. These studies will employ modern stereology, bioassay, immunohistochemistry and in situ hybridization procedures. The results of these investigations will have major implications for therapeutic strategies in aging and MCI and the role of genetics in considering these issues.
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