Ubiquitin over-expression phenotypes and ubiquitin gene molecular misreading during aging in Drosophila melanogaster.

Molecular Misreading (MM) is the inaccurate conversion of genomic information into aberrant proteins. For example, when RNA polymerase II transcribes a GAGAG motif it synthesizes at low frequency RNA with a two-base deletion. If the deletion occurs in a coding region, translation will result in production of misframed proteins. During mammalian aging, misframed versions of human amyloid precursor protein (hApp) and ubiquitin (hUbb) accumulate in the aggregates characteristic of neurodegenerative diseases, suggesting dysfunctional degradation or clearance. Here cDNA clones encoding wild-type hUbb and the frame-shifted version hUbb+1 were expressed in transgenic Drosophila using the doxycycline-regulated system. Misframed proteins were abundantly produced, both from the transgenes and from endogenous Drosophila ubiquitin-encoding genes, and their abundance increased during aging in whole-fly extracts. Over-expression of wild-type hUbb, but not hUbb+1, was toxic during fly development. In contrast, when over-expressed specifically in adult flies, hUbb+1 caused small decreases in life span, whereas hUbb was associated with small increases, preferentially in males. The data suggest that MM occurs in Drosophila and that the resultant misframed proteins accumulate with age. MM of the ubiquitin gene can produce alternative ubiquitin gene products with different and sometimes opposing phenotypic effects.

Four independent germ-line transformants were generated for the hApp construct. hApp [16], [1] and [2]  Tet-on eGFP and DsRED reporter constructs. For the eGFP reporter, PCR products were generated using pGreen Pelican plasmid containing the eGFP gene as a template. The coding region sequences were amplified using primers with a Pst1 site engineered at the 5' end and an EcoRI site engineered at the 3' end. The amplification products were then cloned into the unique PstI and EcoRI sites of USC1.0, to generate the final injection construct. The DsRED reporter construct was generated using the DsRED gene sequences from DsRED Pelican plasmid (pRHP) using analogous procedures.
hApp and hApp +1 Northern and Western analyses. The PCR product APPwt-1 was used as a specific probe for the hApp gene in Northern blot analyses. Western analysis of hApp and hApp +1 employed antibodies purchased from Upstate cell signaling solutions, including Anti-App (Catalog #07-667) as well as antibody specific for hApp +1 ("Amy-5") characterized previously [2]. Additional Western control experiments utilized mouse monoclonal antibody 22c11 (Millipore/Chemicon), specific for the N-terminus of hApp, and cortical neuron lysates as a positive control for App (data not shown).

Analysis of hApp expression and molecular misreading
Human cDNA encoding wild-type hApp protein, and cDNA engineered with the appropriate dinucleotide deletions within the GAGAG motif were cloned downstream of the DOX-regulated promoter (Supplemental Figure S3A,B). These constructs were introduced into Drosophila using P element mediated transformation and multiple independent transgenic strains were generated for each construct. In all the experiments presented, the strains homozygous for the transgenic target constructs were crossed to the rtTA(3)E2 driver strain (or other driver strains, as indicated), to generate hybrid progeny containing both constructs; control flies contained only the rtTA(3)E2 driver construct and no target construct. Expression of hApp in adult male flies was assayed by Western blot, using a specific antibody (Upstate Cat. #07-667). No DOX-inducible species could be detected at the calculated size of ~79Kd, or at other sizes (Supplemental Figure S3D), suggesting that the hApp www.impactaging.com protein is not being expressed at a detectable level and/or is not stable. Other studies have reported that hApp could be expressed in adult flies and detected by Western blot at an apparent MW of ~110Kd [3,4]. One possibility is that hApp is being expressed at low levels in the experiments presented here, but is being obscured by a background band such as the one running at ~100Kd (Supplemental Figure S3D; indicated with asterisk). However DOX inducible expression of hApp was also not detected using mouse monoclonal antibody 22c11, which yielded a different pattern of background bands (data not shown). We conclude that hApp is either not being expressed at a detectable level from this construct in adult male flies, or that the protein is unstable. These hApp constructs are indeed being expressed in a DOX-dependent manner at the RNA level, as confirmed by Northern blots (Supplemental Figure S3C), and as indicated by the fact that they give rise to hApp +1 via apparent MM events, as described next.
To determine if the misframed version of hApp could be detected in flies, Western blots were performed using antibody specific for hApp +1 . The hApp +1 antibody readily detected His-tagged hApp +1 protein purified from E. coli cells, as well as highly abundant protein produced in flies transgenic for the hApp +1 transgenic construct at the same size, consistent with efficient expression of hApp +1 in adult flies ( Figure 5A; indicated by black arrowhead). Notably, both the Histagged hApp +1 and the hApp +1 produced in transgenic flies ran in the gel at a position equivalent to an apparent MW of ~58Kd, which is the reported mobility for hApp +1 under these conditions [5]. This is despite the fact that the calculated MW for the 348 amino acid residue hApp +1 protein is ~39Kd. This unusual retarded mobility in SDS-PAGE gels observed for hApp +1 (as well as hApp) has been observed in several previous studies [5,6], and is attributed to the acidic region of the protein between positions 230-260 that contains many glutamate and aspartate residues. In transgenic flies expressing the hApp transgene, a DOX-inducible band at the same apparent MW of ~58KD was detected, consistent with MM of the hApp transgene (Supplemental Figure S4C, D). It is also interesting to note that there were several species in the Oregon-R control fly extracts that cross-reacted with hApp +1 antibody, including one of a similar size as hApp +1 (indicated by an asterisk), and that these species became more apparent with age (Supplemental Figure S4B). Despite this background, the fact that the apparently ~58Kd species was produced in a DOX-inducible manner in two independent hApp transgenic strains, but not in the controls, suggests that MM is indeed occurring, and moreover that this hApp +1 protein is more readily detected in old flies.
The faint pattern of endogenous Drosophila species cross-reacting with the hApp +1 antibody most likely represents non-specific, cross-reacting proteins, however it is not clear at this time why such crossreactivity is more apparent in old fly extracts. The Drosophila genome contains at least one gene related to hApp, the Appl gene, however it is not obvious how it could encode a cross-reacting epitope or an appropriately sized protein based on its known sequence [3].

SUPPLEMENTAL REFERENCES
Supplemental Figure S1. Nucleotide sequences and translation of the transcripts expected from the transgenic constructs hUbb and hUbb +1 . (A) The hUbb construct sequence and transcript. The sequence of the transgenic construct is presented starting from the TATA box of the promoter through the polyadenylation signal sequence (indicated in bold). The location of the unique PstI and EcoRI cloning sites of the USC1.0 vector are indicated by underline; the EcoRI site is destroyed during cloning. The location of nucleotide +1 of the transcript is indicated with an arrow. The coding region for wild-type ubiquitin is indicated in blue, and the stop codon is indicated in red with an asterisk. The translation of the entire transcript is presented in each of three reading frames. Methionine residues are indicated in blue, and stop codons are indicated with red asterisk. In translation frame 3, the potential partial match to the +1 epitope is indicated in red. (B) The hUbb +1 construct sequence and transcript. The sequence of the transgenic construct is presented starting from the TATA box of the promoter through the polyadenylation signal sequence (indicated in bold). The location of the unique PstI and EcoRI cloning sites of the USC1.0 vector are indicated by underline; the EcoRI site is destroyed during cloning. The location of nucleotide +1 of the transcript is indicated with an arrow. The atg start codon for translation of the first Ubb repeat is indicated in blue bold-face, the corresponding atg sequence in the second repeat is indicated in blue. The gagag hotspot for MM is indicated with yellow highlight. The translation of the transcript is indicated below using single letter amino acid code. Note that this hUbb +1 construct has been engineered to constitutively encode hUbb +1 protein.
This was done by deleting the conserved gt dinucleotide, located immediately downstream of the gagag hotspot, such that misframed translation proceeds into the second Ubb repeat to generate the +1 epitope, which is indicated in red. www.impactaging.com