Identification of novel genes that modify phenotypes induced by Alzheimer's beta-amyloid overexpression in Drosophila

Abstract

Sustained increases in life expectancy have underscored the importance of managing diseases with a high incidence in late life, such as various neurodegenerative conditions. Alzheimer's disease (AD) is the most common among these, and consequently significant research effort is spent on studying it. Although a lot is known about the pathology of AD and the role of beta-amyloid (Abeta) peptides, the complete network of interactions regulating Abeta metabolism and toxicity still eludes us. To address this, we have conducted genetic interaction screens using transgenic Drosophila expressing Abeta and we have identified mutations that affect Abeta metabolism and toxicity. These analyses highlight the involvement of various biochemical processes such as secretion, cholesterol homeostasis, and regulation of chromatin structure and function, among others, in mediating toxic Abeta effects. Several of the mutations that we identified have not been linked to Abeta toxicity before and thus constitute novel potential targets for AD intervention. We additionally tested these mutations for interactions with tau and expanded-polyglutamine overexpression and found a few candidate mutations that may mediate common mechanisms of neurodegeneration. Our data offer insight into the toxicity of Abeta and open new areas for further study into AD pathogenesis.

Figure 1.—

Light stereomicrographs of transgenic Drosophila eyes coexpressing Aβ42 and selected modifier mutations. (A) Aβ42-only expressing flies. (B–F) Enhancers of the Aβ42 eye phenotype. (G–H) Suppressors of the Aβ42 eye phenotype. Shown are flies coexpressing Aβ42 and Dsp1^KG06700 (B), ATP7^EY07895 (C), svr^EP(X)0356 (D), SNFγ^KG10152 (E), CG6767^KG00420 (F), EP(3)3603 (G), or EP(3)3348 (H).

Figure 2.—

Comparison of total and soluble levels of Aβ42 peptides in transgenic flies. Measurements are normalized to the levels of Aβ42 in control flies (Aβ only). Genotypes indicate the mutation that is coexpressed with Aβ42 in each strain. Standard deviation was calculated in cases where at least three experiments of the same genotype were available. Asterisks denote statistically significant changes compared to the respective controls. We considered values with P ≤ 0.05 in measurements of soluble Aβ and P ≤ 0.005 in measurements of total Aβ.

Figure 3.—

Effects of Sin3A-complex mutants on levels of Aβ. (A) Levels of soluble Aβ peptides were normalized relative to levels in control flies (Aβ only). Coexpression of Aβ42 with loss-of-function mutations in Sin3A, Rpd3, HDAC4, and SAP130 results in increased levels of soluble Aβ. Coexpression of a putative gain-of-function mutation in SAP130 [EP(3)3348] results in decreased levels of Aβ. (B) Levels of Aβ42 RNA, measured by quantitative PCR, in control flies (Aβ only) and in flies coexpressing a putative gain-of-function mutation in SAP130 [EP(3)3348]. The presence of the EP(3)3348 mutation does not cause a statistically significant change in levels of Aβ42 RNA.

Figure 4.—

Modifiers of the tau- and httex1^Q93-induced eye phenotypes. (A–D) Comparison of eye phenotypes caused by expression of different disease proteins. (A) Wild-type flies, (B) flies expressing Aβ42, (C) flies expressing tau, (D) flies expressing httex1^Q93. (F and G) Enhancers of the tau phenotype. (H–L) Suppressors of the tau phenotype. Shown are flies expressing only tau (E) or coexpressing tau and EP(X)1318 (F), EP(3)3470 (G), mub^EP(3)3108 (H), EP(3)3405 (I), svr^EP(X)0356 (J), EP(3)3348 (K), or EP(X)0308 (L). (N–P) Enhancers of the httex1^Q93 phenotype. (Q–T) Suppressors of the httex1^Q93 phenotype. Shown are flies expressing only httex1^Q93 (M) or coexpressing httex1^Q93 and Dsp1^EP(X)0355 (N), EP(3)595b (O), SNFγ^EP(3)3015b (P), EP(X)0308 (Q), EP(2)0330 (R), MESR4^EP(2)0386 (S), or CG2924^EP(X)1596 (T).