Quantification assays for total and polyglutamine-expanded huntingtin proteins

Abstract

The expansion of a CAG trinucleotide repeat in the huntingtin gene, which produces huntingtin protein with an expanded polyglutamine tract, is the cause of Huntington's disease (HD). Recent studies have reported that RNAi suppression of polyglutamine-expanded huntingtin (mutant HTT) in HD animal models can ameliorate disease phenotypes. A key requirement for such preclinical studies, as well as eventual clinical trials, aimed to reduce mutant HTT exposure is a robust method to measure HTT protein levels in select tissues. We have developed several sensitive and selective assays that measure either total human HTT or polyglutamine-expanded human HTT proteins on the electrochemiluminescence Meso Scale Discovery detection platform with an increased dynamic range over other methods. In addition, we have developed an assay to detect endogenous mouse and rat HTT proteins in pre-clinical models of HD to monitor effects on the wild type protein of both allele selective and non-selective interventions. We demonstrate the application of these assays to measure HTT protein in several HD in vitro cellular and in vivo animal model systems as well as in HD patient biosamples. Furthermore, we used purified recombinant HTT proteins as standards to quantitate the absolute amount of HTT protein in such biosamples.

Figure 1. Anti-HTT antibody epitopes and analysis by immunoblot.

(A) Diagram representing antibody epitopes on human HTT protein (relative to GenBank accession CAD38447.1). A stretch of glutamine (Q) residues near the N-terminus is expanded in individuals affected by Huntington's disease. Amino-acid 1-92 encoded by exon-1 are shown. Blue, pAb147 antibody epitope on mouse HTT protein (GenBank accession NP_034544). (B) Immunoblot analysis of human large fragment recombinant HTT proteins detected by the indicated anti-huntingtin antibodies. 5 ng of both HTT (1–573) Q23 (lanes 1, 3, 5, 7, 9, 11, 13 and 15) and HTT (1–573) Q73 (lanes 2, 4, 6, 8, 10, 12, 14, and 16) purified large fragment proteins were analyzed by SDS-PAGE. M, molecular weight marker (kDa). (C) Immunoblot analysis of wild type littermate and transgenic BAC HD mouse whole brain extracts. Mouse endogenous wild type and human transgenic polyglutamine-expanded HTT proteins were detected using the indicated anti-huntingtin antibodies. 25 µg of normal (wt) and transgenic (tg) mouse brain extracts were analyzed by SDS-PAGE as indicated. M, molecular weight marker (kDa). Tg mHTT, transgenic human polyglutamine-expanded HTT. WT HTT, endogenous mouse wild type HTT.

Figure 2. MSD assay performance with human and mouse HTT purified proteins.

HTT (1–573) Q23 and HTT (1–573) Q73 large fragment and full length wild type (Q17) and mutant (Q46) recombinant human HTT proteins were spiked in the MSD assay buffer 1 at different concentrations and tested in the expanded polyglutamine human HTT MSD assay (antibody pair pAb146/MW1) (A), in the pan (antibody pair pAb146/MAB2166) (B), and in the exon-1 - pan (antibody pair pAb146/pAb137) (C) human HTT MSD assays. The HTT (1–549) Q7 mouse large fragment recombinant HTT protein was spiked in the MSD assay buffer 1 at different concentrations and tested in the mouse/rat HTT MSD assay (antibody pair pAb147/MAB2166) (D). Data are averages of n = 2 technical replicates with correspondent standard deviations.

Figure 3. Specificity of human mutant and mouse HTT detection.

The adeno-associated AAV-shRNA expression vector AAV-SEWB-sh4 was transduced into heterozygous zQ175 mouse primary neurons and humanized mutant (A) or endogenous mouse (B) HTT proteins were evaluated using the expanded polyglutamine human HTT MSD assay (antibody pair pAb146/MW1) or the mouse/rat HTT MSD assay (antibody pair pAb147/MAB2166), respectively. sh4, HTT targeting shRNA. scr6, scramble control shRNA. (C) Neuronal total tau protein levels measured using a commercially available MSD ELISA-based assay kit were monitored as loading control. Data are averages of n = 3 independent samples with correspondent standard deviations. ***, P<0.001. (D) Immunoblot confirming the AAV-mediated knockdown of humanized mutant (mut) and endogenous mouse HTT in transduced heterozygous zQ175 mouse primary neurons (AAV-SEWB-sh4: HTT targeting shRNA; AAV-SEWB-scr6: scrambled control shRNA). Immunoblot was probed for HTT (MAB2166, 1∶1,000; Millipore) or ATP5B as loading control.

Figure 4. Decrease of soluble mutant HTT levels in R6/2 brain tissues is associated with increased age of the mice.

(A) Homogenates from brains of R6/2 female mice obtained from The Jackson Laboratory (bearing an expanded polyglutamine tract of 120 CAG repeats on average) were analyzed for detection of soluble mutant human HTT at different ages (4, 8 and 12 weeks). Tissues analyzed showed significant signals in the expanded polyglutamine human HTT MSD assay (antibody pair pAb146/MW1) and a significant signal decrease between 4 and 8 weeks of age. Data are averages of n = 4 independent samples with correspondent standard deviations. B, assay background. ***, P<0.001. (B) SDS-PAGE and immunoblotting for HTT with the MW8 antibody reveals high molecular weight bands (presumably HTT aggregates) in R6/2 brain homogenates that increase with increasing age of the animals. Progressive decrease of soluble monomeric HTT fragments can be observed. WT, wild type (CBA×C57Bl/6) F1 (CBF) (B6CBAF1/OlaHsd, Harlan Olac) mice.

Figure 5. Quantification of mutant human HTT and endogenous mouse HTT detection in brain extracts derived from three different mouse HD models.

Human mutant HTT proteins expressed in 3 month-old BAC HD, 4 week-old R6/2 and 3 month-old heterozygous zQ175 mouse models were detected using the expanded polyglutamine human HTT MSD assay (antibody pair pAb146/MW1) (A) and the pan human HTT MSD assay (antibody pair pAb146/MAB2166) for BAC HD and zQ175 mice or the exon-1 - pan human HTT MSD assay (antibody pair pAb146/pAb137) for R6/2 mice (B). A total of 20 µg of whole brain tissue homogenates were used for the analysis. Correspondent wild type age-matched control animals (wt) were included as negative controls. (C) Human mutant HTT proteins in the above mentioned HD mouse models were quantified using standard curves of large fragment (exon-1 - pan human HTT MSD assay) or full length HTT recombinant proteins (pan human HTT MSD assay) as standards. (D) The expression of mouse endogenous HTT was detected using the mouse/rat HTT MSD assay (antibody pair pAb147/MAB2166) and quantified using standard curves of purified HTT (1–549) large fragment mouse HTT protein. Quantified human HTT and mouse HTT proteins are expressed as femtomoles per milligrams of total brain extract input protein. Data are averages of n = 3 independent samples with correspondent standard deviations. B, assay background. *, P<0.05; **, P<0.01; ***, P<0.001.

Figure 6. Detection of polyglutamine-expanded (antibody pair pAb146/MW1) and non-expanded (antibody pair pAb146/MAB2166) human HTT proteins in HD patient lymphoblast cell lines and human brain tissues.

(A) Detection and quantification of human HTT proteins in HD and non-HD control patient lymphoblast lysates. Human HTT proteins were quantified using standard curves of HTT (1–573) Q73 recombinant protein. Technical triplicates with standard deviations are shown. 1 mg/ml of total protein lysates was tested. (B) Detection and quantification of human HTT proteins in post mortem frontal cortex homogenates of 4 HD patient and 3 non-HD control donors. Human HTT proteins were quantified using standard curves of human full length Q46 HTT recombinant protein. Data are presented as average values with correspondent standard deviations. Asterisk marks HTT levels below the limit of detection. 0.5 mg/ml of total protein homogenates was tested. For the analysis of the human brain extracts in the HTT MSD assays, the MW1 detection antibody was used at a concentration of 7.5 µg/ml and the MAB2166 detection antibody was used at a dilution of 1∶1,000.