Polyglutamine expansion alters the dynamics and molecular architecture of aggregates in dentatorubropallidoluysian atrophy

p62-containing, proteolytically active nuclear condensates, increase the efficiency of the ubiquitin-proteasome system

Degradation of a protein by the ubiquitin-proteasome system (UPS) is a multistep process catalyzed by sequential reactions. Initially, ubiquitin is conjugated to the substrate in a process mediated by concerted activity of three enzymes; the last of them-a ubiquitin ligase (E3)-belongs to a family of several hundred members, each recognizing a few specific substrates. This is followed by repeated addition of ubiquitin moieties to the previously conjugated one to generate a ubiquitin chain that serves as a recognition element for the proteasome, which then degrades the substrate. Ubiquitin is recycled via the activity of deubiquitinating enzymes (DUBs). It stands to reason that efficiency of such a complex process would depend on colocalization of the different components in an assembly that allows the reactions to be carried out sequentially and processively. Here we describe nuclear condensates that are dynamic in their composition. They contain p62 as an essential component. These assemblies are generated by liquid-liquid phase separation (LLPS) and also contain ubiquitinated targets, 26S proteasome, the three conjugating enzymes, and DUBs. Under basal conditions, they serve as efficient centers for proteolysis of nuclear proteins (e.g., c-Myc) and unassembled subunits of the proteasome, suggesting they are involved in cellular protein quality control. Supporting this notion is the finding that such foci are also involved in degradation of misfolded proteins induced by heat and oxidative stresses, following recruitment of heat shock proteins and their associated ubiquitin ligase CHIP.

An Expanded CAG Repeat in Huntingtin Causes +1 Frameshifting

Maintenance of triplet decoding is crucial for the expression of functional protein because deviations either into the -1 or +1 reading frames are often non-functional. We report here that expression of huntingtin (Htt) exon 1 with expanded CAG repeats, implicated in Huntington pathology, undergoes a sporadic +1 frameshift to generate from the CAG repeat a trans-frame AGC repeat-encoded product. This +1 recoding is exclusively detected in pathological Htt variants, i.e. those with expanded repeats with more than 35 consecutive CAG codons. An atypical +1 shift site, UUC C at the 5' end of CAG repeats, which has some resemblance to the influenza A virus shift site, triggers the +1 frameshifting and is enhanced by the increased propensity of the expanded CAG repeats to form a stem-loop structure. The +1 trans-frame-encoded product can directly influence the aggregation of the parental Htt exon 1.

Identification of novel polyglutamine-expanded aggregation species in spinal and bulbar muscular atrophy

Polyglutamine-repeat disorders are part of a larger family of neurodegenerative diseases characterized by protein misfolding and aggregation. In spinal and bulbar muscular atrophy (SBMA), polyglutamine expansion within the androgen receptor (AR) causes progressive debilitating muscular atrophy and lower motor neuron loss in males. Although soluble polyglutamine-expanded aggregation species are considered toxic intermediates in the aggregation process, relatively little is known about the spectrum of structures that are formed. Here we identify novel polyglutamine-expanded AR aggregates that are SDS-soluble and bind the toxicity-predicting antibody 3B5H10. Soluble, 3B5H10-reactive aggregation species exist in low-density conformations and are larger by atomic force microscopy, suggesting that they may be less compact than later-stage, insoluble aggregates. We demonstrate disease-relevance in vivo and draw correlations with toxicity in vitro. This article is part of a Special Issue entitled SI: Neuroprotection.

Somatic expression of unc-54 and vha-6 mRNAs declines but not pan-neuronal rgef-1 and unc-119 expression in aging Caenorhabditis elegans

Aging is a highly controlled biological process characterized by a progressive deterioration of various cellular activities. One of several hallmarks of aging describes a link to transcriptional alteration, suggesting that it may impact the steady-state mRNA levels. We analyzed the mRNA steady-state levels of polyCAG-encoding transgenes and endogenous genes under the control of well-characterized promoters for intestinal (vha-6), muscular (unc-54, unc-15) and pan-neuronal (rgef-1, unc-119) expression in the nematode Caenorhabditis elegans. We find that there is not a uniform change in transcriptional profile in aging, but rather a tissue-specific difference in the mRNA levels of these genes. While levels of mRNA in the intestine (vha-6) and muscular (unc-54, unc-15) cells decline with age, pan-neuronal tissue shows more stable mRNA expression (rgef-1, unc-119) which even slightly increases with the age of the animals. Our data on the variations in the mRNA abundance from exemplary cases of endogenous and transgenic gene expression contribute to the emerging evidence for tissue-specific variations in the aging process.

Stable polyglutamine dimers can contain β-hairpins with interdigitated side chains-but not α-helices, β-nanotubes, β-pseudohelices, or steric zippers

A common thread connecting nine fatal neurodegenerative protein aggregation diseases is an abnormally expanded polyglutamine tract found in the respective proteins. Although the structure of this tract in the large mature aggregates is increasingly well described, its structure in the small early aggregates remains largely unknown. As experimental evidence suggests that the most toxic species along the aggregation pathway are the small early ones, developing strategies to alleviate disease pathology calls for understanding the structure of polyglutamine peptides in the early stages of aggregation. Here, we present a criterion, grounded in available experimental data, that allows for using kinetic stability of dimers to assess whether a given polyglutamine conformer can be on the aggregation path. We then demonstrate that this criterion can be assessed using present-day molecular dynamics simulations. We find that although the α-helical conformer of polyglutamine is very stable, dimers of α-helices lack the kinetic stability necessary to support further oligomerization. Dimers of steric zipper, β-nanotube, and β-pseudohelix conformers are also too short-lived to initiate aggregation. The β-hairpin-containing conformers, instead, invariably form very stable dimers when their side chains are interdigitated. Combining these findings with the implications of recent solid-state NMR data on mature fibrils, we propose a possible pathway for the initial stages of polyglutamine aggregation, in which β-hairpin-containing conformers act as templates for fibril formation.

The L-cysteine desulfurase NFS1 is localized in the cytosol where it provides the sulfur for molybdenum cofactor biosynthesis in humans

In humans, the L-cysteine desulfurase NFS1 plays a crucial role in the mitochondrial iron-sulfur cluster biosynthesis and in the thiomodification of mitochondrial and cytosolic tRNAs. We have previously demonstrated that purified NFS1 is able to transfer sulfur to the C-terminal domain of MOCS3, a cytosolic protein involved in molybdenum cofactor biosynthesis and tRNA thiolation. However, no direct evidence existed so far for the interaction of NFS1 and MOCS3 in the cytosol of human cells. Here, we present direct data to show the interaction of NFS1 and MOCS3 in the cytosol of human cells using Förster resonance energy transfer and a split-EGFP system. The colocalization of NFS1 and MOCS3 in the cytosol was confirmed by immunodetection of fractionated cells and localization studies using confocal fluorescence microscopy. Purified NFS1 was used to reconstitute the lacking molybdoenzyme activity of the Neurospora crassa nit-1 mutant, giving additional evidence that NFS1 is the sulfur donor for Moco biosynthesis in eukaryotes in general.

Assessing polyglutamine conformation in the nucleating event by molecular dynamics simulations

Polyglutamine (polyQ) diseases comprise a group of dominantly inherited pathology caused by an expansion of an unstable polyQ stretch which is presumed to form β-sheets. Similar to other amyloid pathologies, polyQ amyloidogenesis occurs via a nucleated polymerization mechanism, and proceeds through energetically unfavorable nucleus whose existence and structure are difficult to detect. Here, we use atomistic molecular dynamics simulations in explicit solvent to assess the conformation of the polyQ stretch in the nucleus that initiates polyQ fibrillization. Comparison of the kinetic stability of various structures of polyQ peptide with a Q-length in the pathological range (Q40) revealed that steric zipper or nanotube-like structures (β-nanotube or β-pseudohelix) are not kinetically stable enough to serve as a template to initiate polyQ fibrillization as opposed to β-hairpin-based (β-sheet and β-sheetstack) or α-helical conformations. The selection of different structures of the polyQ stretch in the aggregation-initiating event may provide an alternative explanation for polyQ aggregate polymorphism.