Mechanisms underlying Abeta42-induced neuronal dysfunction and degeneration
Deposits of 42 amino acid amyloid-beta peptide (Abeta42) and hyper-phosphorylated microtubule-associated protein tau in the brain are the pathological hallmarks of Alzheimer's disease (AD). Accumulating evidence suggests that Abeta42 lies upstream of tau in a pathological cascade. However, two critical questions remain elusive. First, how does Abeta42 induce pathological consequences in AD? Second, how does Abeta42 induce abnormal phosphorylation and toxicity of tau? Addressing these questions will advance our understanding of complex AD pathogenesis, and lead to the discovery of novel therapeutic interventions. In this study, Drosophila is used as an efficient genetic model system to unravel molecular mechanism underlying Abeta42-induced toxicity in vivo. Using gene expression profiling and a genome-wide screen, the genes/pathways and chromosomal loci that modify Abeta42-induced neuronal dysfunction were identified. Moreover, co-expression of Abeta42 enhanced phosphorylation and toxicity of tau in fly eyes and brains. Based on these data, we designed our specific aims as follows; Specific Aim 1: To examine the role of and mechanisms underlying reduced cAMP/PKA/CREB activity in Abeta42 toxicity. The cAMP/PKA/CREB pathway plays critical roles in the execution and maintenance of complex brain functions such as memory formation and energy metabolism, and the dysregulation of this pathway has been implicated in the pathogenesis of AD. However, it is not fully understood 1) how Abeta42 affects PKA and CREB activity and 2) whether activation of this pathway protects against Abeta42-induced toxicity in vivo. The relationship of Abeta42 toxicity to dysregulation in the cAMP/PKA/CREB pathway will be analyzed in Abeta42 flies. Specific Aim 2: To elucidate molecular mechanisms underlying toxic interactions between microtubule associated protein tau and Abeta42. Our preliminary data showed that co-expression of A242 and tau enhanced tau-induced toxicity in the eyes and brains of flies, which correlates with increased tau phosphorylation at the AD-related sites S202, T231, and S262. Interestingly, tau phosphorylation at T231 and S262 is known to be upregulated in pretangle neurons in AD brains. Moreover, phosphorylation at S262 has been shown to promote both tau phosphorylation at other sites and tau toxicity. The mechanisms underlying the enhancement of tau phosphorylation and toxicity by Abeta42 will be studied using our fly model as a genetic model system, which may recapitulate an initial step in the abnormal metabolism of tau in AD brains. Specific Aim 3: A search for genes that modify the neuronal dysfunction induced by Abeta42. By a genetic screen, ten chromosomal loci whose haploinsufficiency enhances A2beta2-induced behavioral deficits were identified. The modifier genes in these loci will be identified and characterized. This study will provide mechanistic insights into how Abeta42 induces neurotoxicity and tau pathology in vivo, which will facilitate our understanding of AD pathogenesis and may open novel therapeutic avenues for AD.