Native mutant huntingtin in human brain: evidence for prevalence of full-length monomer.
ABSTRACT Huntington disease (HD) is caused by polyglutamine expansion in the N terminus of huntingtin (htt). Analysis of human postmortem brain lysates by SDS-PAGE and Western blot reveals htt as full-length and fragmented. Here we used Blue Native PAGE (BNP) and Western blots to study native htt in human postmortem brain. Antisera against htt detected a single band broadly migrating at 575-850 kDa in control brain and at 650-885 kDa in heterozygous and Venezuelan homozygous HD brains. Anti-polyglutamine antisera detected full-length mutant htt in HD brain. There was little htt cleavage even if lysates were pretreated with trypsin, indicating a property of native htt to resist protease cleavage. A soluble mutant htt fragment of about 180 kDa was detected with anti-htt antibody Ab1 (htt-(1-17)) and increased when lysates were treated with denaturants (SDS, 8 M urea, DTT, or trypsin) before BNP. Wild-type htt was more resistant to denaturants. Based on migration of in vitro translated htt fragments, the 180-kDa segment terminated ≈htt 670-880 amino acids. If second dimension SDS-PAGE followed BNP, the 180-kDa mutant htt was absent, and 43-50 kDa htt fragments appeared. Brain lysates from two HD mouse models expressed native full-length htt; a mutant fragment formed if lysates were pretreated with 8 M urea + DTT. Native full-length mutant htt in embryonic HD(140Q/140Q) mouse primary neurons was intact during cell death and when cell lysates were exposed to denaturants before BNP. Thus, native mutant htt occurs in brain and primary neurons as a soluble full-length monomer.
SourceAvailable from: Andrea Caricasole[Show abstract] [Hide abstract]
ABSTRACT: Huntington's disease (HD) is a monogenic disorder caused by an aberrant expansion of CAG repeats in the huntingtin gene (HTT). Pathogenesis is associated with expression of the mutant (mHTT) protein in the CNS, with its levels most likely related to disease progression and symptom severity. Since non-invasive methods to quantify HTT in the CNS do not exist, measuring amount of soluble HTT in peripheral cells represents an important step in development of disease-modifying interventions in HD. An ELISA assay using commercially available antibodies was developed to quantify HTT levels in complex matrices like mammalian cell cultures lysates and human samples. The immunoassay was optimized using a recombinant full-length HTT protein, and validated both on wild-type and mutant HTT species. The ability of the assay to detect significant variations of soluble HTT levels was evaluated using an HSP90 inhibitor that is known to enhance HTT degradation. Once optimized, the bioassay was applied to peripheral blood mononuclear cells (PBMCs) from HD patients, demonstrating good potential in tracking the disease course. The method described here represents a validated, simple and rapid bio-molecular assay to evaluate soluble HTT levels in blood cells as useful tool in disease and pharmacodynamic marker identification for observational and clinical trials.BMC Biochemistry 11/2013; 14(1):34. DOI:10.1186/1471-2091-14-34 · 1.94 Impact Factor
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ABSTRACT: Huntington's disease (HD) represents an important model for neurodegenerative disorders and proteinopathies. It is mainly caused by cytotoxicity of the mutant huntingtin protein (Htt) with an expanded polyQ stretch. While Htt is ubiquitously expressed, HD is characterized by selective neurodegeneration of the striatum. Here we report a striatal-enriched orphan G protein-coupled receptor(GPCR) Gpr52 as a stabilizer of Htt in vitro and in vivo. Gpr52 modulates Htt via cAMP-dependent but PKA independent mechanisms. Gpr52 is located within an intron of Rabgap1l, which exhibits epistatic effects on Gpr52-mediated modulation of Htt levels by inhibiting its substrate Rab39B, which co-localizes with Htt and translocates Htt to the endoplasmic reticulum. Finally, reducing Gpr52 suppresses HD phenotypes in both patient iPS-derived neurons and in vivo Drosophila HD models. Thus, our discovery reveals modulation of Htt levels by a striatal-enriched GPCR via its GPCR function, providing insights into the selective neurodegeneration and potential treatment strategies.
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ABSTRACT: Classical targeted drug discovery is based on targeting druggable targets, typically kinases and receptors of which the function can be agonized or antagonized. This strategy meets difficulties in cases such as Huntington's disease (HD) and similar neurodegenerative disorders, where the pathological function of the protein causing the disease is not clear. HD is caused by mutant HTT protein (mHTT) containing an expanded polyglutamine (polyQ) stretch, but the function of mHTT and how mHTT causes HD are unknown, thus preventing efforts to screen for mHTT 'inhibitors'. However, HD is appealing for drug discovery because the genetic mutation is clear, as compared with other major neurodegenerative disorders. Although mHTT is not a conventional 'druggable' target, one approach that appears promising is lowering its level, which might be applicable to other neurodegenerative disorders and proteinopathies linked to aberrant accumulation of proteins. Here we review mHTT lowering strategies that might provide promising avenues for drugging such diseases.Trends in Pharmacological Sciences 01/2014; DOI:10.1016/j.tips.2013.12.001 · 9.99 Impact Factor