Quantitation of BDNF mRNA in human parietal cortex by competitive reverse transcription-polymerase chain reaction: decreased levels in Alzheimer's disease

Abstract

Alzheimer's disease is a progressive neurodegenerative disorder of the central nervous system. One pathological characteristic is excessive neuronal loss in specific regions of the brain. Among the areas most severely affected are the basal forebrain cholinergic neurons and their projection regions, the hippocampus and cortex. Neurotrophic factors, particularly the neurotrophins nerve growth factor and brain-derived neurotrophic factor, play an important role in the development, regulation and survival of basal forebrain cholinergic neurons. Furthermore, brain-derived neurotrophic factor regulates the function of hippocampal and cortical neurons. Neurotrophins are synthesized in hippocampus and cortex and retrogradely transported to the basal forebrain. Decreased levels of neurotrophic factors are suspected to be involved in the neurodegenerative changes observed in Alzheimer's disease. We examined autopsied parietal cortex samples from age- and gender-matched Alzheimer's diseased and neurologically non-impaired individuals using the quantitative technique of competitive RT-PCR. We demonstrate a 3.4-fold decrease in brain-derived neurotrophic factor mRNA levels in the parietal cortex of patients with Alzheimer's disease compared to controls (p<0.004). A decrease in brain-derived neurotrophic factor synthesis could have detrimental effects on hippocampal, cortical and basal forebrain cholinergic neurons and may account for their selective vulnerability in Alzheimer's disease.

Introduction

Alzheimer's disease (AD), a neurodegenerative disorder defined by a distinctive clinical course and pathology, is characterized by neuronal damage and loss of function. Neuronal damage and loss are most severe in the basal forebrain cholinergic neurons followed by the hippocampus, the amygdala and neocortical regions 6, 9, 28, 41. The basal forebrain cholinergic neurons consist of a group of neurons with projections to the hippocampus and neocortical areas of the brain. Atrophy and loss of basal forebrain cholinergic neurons, cells that are critically involved in functions of memory and cognition [15], correlate strongly with the degree of dementia in AD [5].

Neurotrophins are a subclass of neurotrophic factors and, in mammals, include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3, and neurotrophin-4/5. These factors are synthesized in many regions of the brain and are important for neuronal survival, function and regulation 7, 8, 13, 15, 25, 27, 34, 40. Both NGF and BDNF support the function and survival of the basal forebrain cholinergic neurons 1, 14, 36which has led to the suggestion that these factors are important in the etiology of AD.

Studies on the relative levels of BDNF mRNA and protein in AD have provided conflicting results. Decreased BDNF mRNA levels have been observed in the AD hippocampus 32, 35and entorhinal cortex [35]whereas other cortical structures, including parietal [17], have shown no deficit when compared to control tissue samples. Narisawa-Saito and colleagues [33]using enzyme immunoassays (EIA) as a method of protein detection, have shown region-specific alterations of BDNF in AD: depleted levels were observed in the entorhinal cortex but not in the hippocampal dentate gyrus or motor cortex. Using immunohistochemistry, Connor et al. [4]reported decreased BDNF protein in AD temporal cortex and hippocampus, while Ferrer et al. [10]reported decreased BDNF protein in AD frontal cortex by Western blotting.

In this study, we examined parietal cortical samples from AD patients and age- and gender-matched, neurologically normal subjects. We used quantitative competitive RT-PCR, a more quantitative technique than those used previously to measure BDNF mRNA.