The cognitive alterations observed in Alzheimer's disease (AD) might be the result of an extensive loss of cortico-cortico afferent combined with an aberrant sprouting in the areas of synapse loss. We hypothesize that in those areas the dystrophic neurites scattered in the neuropil and in the plaques originate in the abnormal axons and presynaptic terminals, as the result of the aberrant sprouting where amyloid precursor protein (APP) and growth associated protein (GAP-43) may play a role. In order to understand the subcellular basic and mechanisms of response to synapse loss and pathology in AD and its relationship to the neuritic plaque formation, this project will utilize two approaches. One is to perform 3-D reconstructions of synapses, scattered dystrophic neurites and neuritic plaques in AD cortex and the other is to develop an experimental model of denervation with widespread dystrophic changes. The 3-D reconstructions will be performed from serial semithin sections obtained from AD frontal cortex biopsies processed with a Silicon Graphics Iris 4D/210VGX workstation. Reconstructions will also be performed from optical sections obtained from the laser confocal imaging of double immunolabeled sections of the AD frontal cortex. The sections will be labeled with fluorescent tagged antibodies that recognize presynaptic terminals and dystrophic neurites with anti-synaptophysin and tau respectively. We also will analyze the role of APP and GAP-43 in the sprouting response in the AD cortex by comparing the patterns of APP-immunoreactive synapses and dystrophic neurites to those of GAP-43 and synaptophysin in double immunolabeled sections. The quantitative information derived from these studies and 3-D images of the AD neuropil and neuritic plaques are expected to provide important information about the relationship between the pathological synapses in the neuropil and in the plaques with the dystrophic neurites. The other approach that we propose is to develop an animal model of neocortical dystrophic changes. For this study, rats will receive unilateral aspirative lesion of the fronto-parietal cortex followed by the injection of a neurotoxin that will alter the process of plasticity. The rationale is that AD pathogenesis may be a multi-step process in which several sequential hits are necessary to damage the repair mechanisms in the cortex. In the first step, there is a widespread synapse loss in cortex, followed by a neurotoxic event that eventually will lead to an ineffective and abnormal repair process in the denervated regions. The main neurotoxin we will test is the phorbol 12-myristate 13-acetate (PMA). The rationale behind using PMA in our model of denervation, is that it will interfere with reinnervation, promote an aberrant sprouting response similar to the one observed in AD, affect PKC and GAP-43, and promote the expression of amyloid genes in rat. We expect that this lesion and neurotoxin combination will reproduce, in the rodent model, the widespread dystrophic alterations observed in AD and provide information about the pathogenesis of the synaptic alterations observed in AD.
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