Aggregation landscapes of Huntingtin exon 1 protein fragments and the critical repeat length for the onset of Huntington’s disease

Abstract

Huntington's disease (HD) is a neurodegenerative disease caused by an abnormal expansion in the polyglutamine (polyQ) track of the Huntingtin (HTT) protein. The severity of the disease depends on the polyQ repeat length, arising only in patients with proteins having 36 repeats or more. Previous studies have shown that the aggregation of N-terminal fragments (encoded by HTT exon 1) underlies the disease pathology in mouse models and that the HTT exon 1 gene product can self-assemble into amyloid structures. Here, we provide detailed structural mechanisms for aggregation of several protein fragments encoded by HTT exon 1 by using the associative memory, water-mediated, structure and energy model (AWSEM) to construct their free energy landscapes. We find that the addition of the N-terminal 17-residue sequence (NT 17 ) facilitates polyQ aggregation by encouraging the formation of prefibrillar oligomers, whereas adding the C-terminal polyproline sequence (P 10 ) inhibits aggregation. The combination of both terminal additions in HTT exon 1 fragment leads to a complex aggregation mechanism with a basic core that resembles that found for the aggregation of pure polyQ repeats using AWSEM. At the extrapolated physiological concentration, although the grand canonical free energy profiles are uphill for HTT exon 1 fragments having 20 or 30 glutamines, the aggregation landscape for fragments with 40 repeats has become downhill. This computational prediction agrees with the critical length found for the onset of HD and suggests potential therapies based on blocking early binding events involving the terminal additions to the polyQ repeats.Huntington's disease | aggregation | solubility | aggregation free energy landscape | critical length H untingtin (HTT) is a huge protein (3,100 amino acid residues) that has been implicated in a variety of physiological functions (1, 2). Having an expanded polyglutamine (polyQ) region of 36 or more glutamine residues in the N terminus of HTT leads to Huntington's disease as witnessed by the deposition of large intracellular protein aggregates or inclusion bodies, which are comprised primarily of N-terminal fragments coded by the exon 1 sequence of the mutant HTT (2-5). These N-terminal fragments, each composed of an N-terminal 17-residue sequence (NT17), a polyQ sequence that has expanded in length, and a proline-rich region afterward, originate from proteolysis of HTT (2, 3). The aggregation of the HTT exon 1 product is, therefore, believed to cause Huntington's disease. Clinical studies have shown an inverse correlation between polyQ length and age of disease onset (5). By implicating polymer physics in the disease process, this regularity has intrigued biophysicists for decades.Expanded polyQ sequences, more generally, are associated with nine known neurodegenerative diseases (4). Biophysical chemists have, therefore, focused on first understanding the aggregation of pure polyQ peptides. The rate of aggregation of polyQ peptides is length-dependent, and primary nucleation is rate-limiti...

Effects of flanking sequences and cellular context on subcellular behavior and pathology of mutant HTT

“…1) to various proteolytic fragments exhibiting varying degrees of much-reduced toxicity (18). These observations are consistent with studies showing that the physical properties of HTT proteins are strongly influenced by amino acids surrounding the expanded polyQ (60,64). Thus, the degree of polyQ toxicity is highly sensitive to peptide context.…”

Proteostasis in Huntington's disease: disease mechanisms and therapeutic opportunities

“…Exon 1 is comprised of the N-terminal 17 amino acids (N17), the polyQ tract and then a 51-residue proline-rich domain (PRD). The structure of N17 is α-helical and the exon 1 protein adopts a condensed, disordered state at high concentrations [33] . The exon 1 structure is altered by polyQ expansion although there is no consensus on which morphology of exon 1 represents the toxic aggregate species.…”

Thermodynamics of Homopeptide Aggregation

“…This region has the sequence P 11 -QLPQPPPQAQPLLPQPQ-P 10 . Given the limited flexibility of proline and the propensity to form polyproline helices, this tail will be more rigid, although largely disordered (24). We assume that the tails interact primarily by excluded volume interactions.…”

“…Htt fibrils consist of a cross-β core that spans the polyQ region but does not include N17 or C38 (15,24). Evidence suggests that the β -sheet core is most likely anti-parallel as shown in Fig.…”