Alzheimer's disease (AD) is characterized by three main pathologies: amyloid deposition, tauopathy and chronic inflammation. Even though the primary trigger for the inflammatory events is assumed to be accumulation of A? peptides or the downstream consequences thereof, there appears to be important feed- forward of the inflammatory mediators on the production of A?. Therefore, understanding this cycle of events is of central importance to the development of new therapeutic approaches. It is now widely accepted that the CD40-CD40L dyad is activated upon pro-inflammatory stimulation, which further enhances the inflammatory response. We have previously demonstrated a significant constitutive CNS role of CD40-CD40 interaction in AD pathogenesis. For instance, in a transgenic mouse model of AD, we have shown that CD40/CD40L interaction is required to observe the full complement of AD pathology. We have shown that in the absence of either CD40 or CD40L, there is a drastic reduction in soluble and insoluble amyloid accumulation with a concomitant decrease in inflammation, tau-related pathologies and cognitive impairment in AD mouse models. These studies are further supported by our in vitro data showing CD40L is able to up-regulate A? production in a variety of scenarios. We have recently demonstrated that up-regulation of TRAF dependent NF(B activity after CD40 ligation results in increased generation of A?, most probably by increasing BACE activity. In the absence of normal TRAF signaling, A? production is up-regulated after CD40L stimulation, which suggests that TRAF- independent mechanisms are also involved in APP processing. There is ample circumstantial evidence that lipid rafts may play a significant role in amyloidogenic processing of APP and both (-secretase and ?-secretase have been localized to lipid rafts. It is also known that CD40 triggered protein tyrosine kinase activation takes place within lipid rafts and our preliminary data also show that inhibition of normal raft structure by anthocyanins can inhibit CD40/CD40L mediated production of A?. Thus, in this proposal, we aim to understand TRAF independent mechanisms linking CD40 internalization to amyloidogenic APP processing. In this context, we will examine the roles of CD40 internalization on APP and its metabolites as well as the proteases responsible for the amyloidogenic processing. We have also demonstrated that CD40/CD40L produce human phosphorylated tau oligomers previously associated with cognitive impairment in mouse models of the disease. A major goal of this proposal is to determine which kinases mediate what we have shown to be constitutive CD40 signaling of tau phosphorylation and which ones mediate hyperphosphorylation in the model AD brain. We will therefore investigate the role of protein tyrosine kinase and JAK/STAT signaling pathways (some of which are TRAF- independent) in the mediation of both tau phosphorylation and APP processing. Using a variety of in vitro, ex vivo and in vivo approaches, we will analyze whether different manipulations of pathway components (inhibition or induced activity) will affect amyloidogenic processing of APP and tau phosphorylation status. Together these studies will identify which CD40-TRAF independent and NF(B dependent or independent pathways also play a role in APP processing and whether modulation of these pathways is suitable for AD intervention. The critical final in vivo studies will chronically inhibit CD40 downstream signaling molecules validated in vitro and in CD40 deficient models of cerebral amyloidosis and tau pathology characteristic of AD.
Over 37 percent of the veteran population is 65 years old or older, compared with 13 percent of the general population and therefore they are at increased risk from Alzheimer's disease (AD) and young veterans may be at increased risk for AD by virtue of the high prevalence of traumatic brain injury in this population. We have sought therapeutic targets for novel drug development that can regulate both amyloid and tau pathologies which are both central to AD. We have identified CD40 signaling as a potentially key modulator of both amyloid and tau pathology and this proposal seeks to identify downstream molecules which are more tractable from a drug development perspective.
