APP-Containing Synaptic Organelles in Alzheimer's Disease

Accumulation of an amyloid protein, Aß, is an invariant feature of Alzheimer’s disease (AD). Aß is derived by proteolytic cleavage of a large precursor, the amyloid Aß protein precursor (APP). It is widely believed that at least some forms of AD are caused through the mismetabolism of APP and/or the accumulation of Aß. A preponderance of evidence suggests that neurons are a major source of Aß in the brain. However, there is a surprising lack of knowledge concerning the trafficking of APP in neurons. For example, we do not know which membrane trafficking pathways are involved in the trafficking of APP. This is particularly striking because, as mentioned above, neurons appear to be a primary source of Aß, and the trafficking of APP appears to be an important step in Aß formation. For an understanding of Alzheimer pathogenesis it is therefore imperative to precisely understand the components of membrane trafficking that bring APP to and from the plasma membranes of neurons. Furthermore, without such knowledge, the functional importance of APP trafficking and processing in neurons will remain elusive. The questions which this application proposes to address are focused on filling this knowledge gap, particularly as related to the trafficking and processing of APP in neuronal synapses (i.e., nerve terminals and dendrites). The specific problems which will be addressed are: 1) is APP trafficking associated with the known pathways of synaptic membrane trafficking or are novel pathways involved; and, 2) where in the pathway of neuronal APP trafficking does proteolysis occur? In preliminary studies the applicant has been able to demonstrate that a majority of APP in synapses is found in a novel organelle. Thus, the data represent a first identification and partial purification of a novel synaptic organelle; this organelle may be important in synaptic and/or APP function, and may contribute to APP processing and/or Aß formation. The major focus of this application will be the further purification and characterization of this organelle and the characterization of the organelle as a site of APP metabolism and Alzheimer pathogenesis.     The hypotheses are as follows: 1. APP-containing organelles are involved in Alzheimer pathogenesis. 2. APP-containing organelles are involved in the metabolism of APP.     These hypotheses will be tested with the following specific aims: 1. To test purified APP-containing synaptic organelles from rat brain and from Alzheimer brain for the presence of marker proteins as well as for the Alzheimer-associated proteins, presenilin-1, presenilin-2, and apolipoprotein E. 2. To test purified APP-containing synaptic organelles from rat brain and from Alzheimer brain for their ability to metabolize APP (i.e., to produce secreted APP and/or Aß from intact APP) and for the presence of proteases.