Proteins Containing Expanded Polyglutamine Tracts and Neurodegenerative Disease

Abstract

Several hereditary neurological and neuromuscular diseases are caused by an abnormal expansion of trinucleotide repeats. To date, there have been 10 of these trinucleotide repeat disorders associated with an expansion of the codon CAG encoding glutamine (Q). For these polyglutamine (polyQ) diseases, there is a critical threshold length of the CAG repeat required for disease, and further expansion beyond this threshold is correlated with age of onset and symptom severity. PolyQ expansion in the translated proteins promotes their self-assembly into a variety of oligomeric and fibrillar aggregate species that accumulate into the hallmark proteinaceous inclusion bodies associated with each disease. Here, we review aggregation mechanisms of proteins with expanded polyQ-tracts, structural consequences of expanded polyQ ranging from monomers to fibrillar aggregates, the impact of protein context and post-translational modifications on aggregation, and a potential role for lipid membranes in aggregation. As the pathogenic mechanisms that underlie these disorders are often classified as either a gain of toxic function or loss of normal protein function, some toxic mechanisms associated with mutant polyQ tracts will also be discussed.

Figure 1. Schematic representation of various proteins associated with CAG-disorders

A, CACNA1A, in addition to the polyQ tract, contains four homologous domains (I–IV), each with six transmembrane segments. There is a SNARE interacting site between domains II and III. B, Androgen receptor, in addition to the polyQ tract, contains an N-terminal domain (NTD), a DNA-binding domain (DBD), a hinge region, and a ligand-binding domain (LBD). C, Ataxin-1 contains a polyQ tract and an AXH domain. D, Ataxin-2, contains the polyQ domain, a Like RNA splicing domain Sm1 and Sm2 (Lsm), a Like-Sm-associated domain (LsmAD), and a poly (A)-binding protein interacting motif 2 (PAM2). E, Ataxin-3 contains an N-terminal Josephin domain, three Ub-interacting motifs (UIM), and the polyQ domain. F, The N-terminal region of the Tata-box binding protein consists of four domains (I-IV) followed by a core region. The polyQ region is domain II. G, Ataxin-7 contains a polyQ domain and a putative nuclear localization signal (NLS). H, Atrophin-1, in addition to the polyQ tract, contains two arginine-glutamic acid dipeptide repeats (RE repeats), a nuclear receptor interacting domain (NRI domain), and a highly conserved Atro-box domain. I, The full-length htt protein contains several HEAT repeats. The inset indicates the location of htt exon1, with the N17, polyQ, and proline-rich domains indicated.

Figure 2. A schematic model for misfolding and aggregation of proteins containing expanded polyQ tracts

Monomeric proteins (shown in green) can sample a variety of distinct conformations, with the relative number and stability of these conformers potentially being polyQ length-dependent. Some of these monomeric conformation are aggregation prone and may lead to distinct aggregation pathway, some on pathway (shown in blue) to fibril formation and some off pathway (shown in red) to fibril formation. There appear to be two generic aggregation mechanisms toward fibril formation: (A) a monomeric critical nucleus that leads directly to fibrils and (B) fibril formation via oligomeric intermediates. Protein context of the polyQ domain can influence which pathway is dominant, and flanking sequences may facilitate the formation of some oligomeric intermediates. These two mechanisms are not necessarily mutually exclusive. (C) There are also several off-pathway aggregation routes that can lead to distinct oligomers of various sizes, a variety of annular aggregates, and large amorphous structures. (D) All of these higher order aggregates may accumulate together to form the large inclusions that are hallmarks of polyQ diseases.

Figure 3. Atomic force microscopy images of a variety of aggregates formed by htt-exon1 proteins

(A) Htt exon1 can form a variety of globular, oligomeric species (purple arrows indicate oligomers ~5 nm in height; green arrows indicate oligomers ~10 nm in height). (B) Two morphologically distinct fibrils structures formed by exon1, (orange circles indicates thinner, smooth fibril structures; pink circles indicate thicker fibrils with a beaded morphology). (C) Blue circles indicate large bundles of htt exon1 fibrils. (D) Large, amorphous aggregates of htt exon1 are indicated by red arrows (note the color scale goes up to 50 nm). (E) When htt exon1 aggregates on a lipid bilayer, a variety of oligomeric aggregates (blue arrows) associated with regions of increased membrane roughness (outlined with the green dashes lines) are observed. (F) When adding monomeric htt exon1 to pre-formed fibrils, the monomer can accumulate around the fibrils and form a variety of branching points (numbers indicated the same fibril at 0 minutes and 120 minutes after exposure to monomeric htt exon1).

Figure 4

Toxic gain of function pathogenic mechanisms associated with polyQ diseases.