Full length huntingtin is not detected in intranuclear inclusions in Huntington's disease brain

The Role of Liquid-Liquid Phase Separation in the Accumulation of Pathological Proteins: New Perspectives on the Mechanism of Neurodegenerative Diseases

It is widely accepted that living organisms form highly dynamic membrane-less organelles (MLOS) with various functions through phase separation, and the indispensable role that phase separation plays in the mechanisms of normal physiological functions and pathogenesis is gradually becoming clearer. Pathological aggregates, regarded as hallmarks of neurodegenerative diseases, have been revealed to be closely related to aberrant phase separation. Specific proteins are assembled into condensates and transform into insoluble inclusions through aberrant phase separation, contributing to the development of diseases. In this review, we present an overview of the progress of phase separation research, involving its biological mechanisms and the status of research in neurodegenerative diseases, focusing on five main disease-specific proteins, tau, TDP-43, FUS, α-Syn and HTT, and how exactly these proteins reside within dynamic liquid-like compartments and thus turn into solid deposits. Further studies will yield new perspectives for understanding the aggregation mechanisms and potential therapeutic strategies, and future research directions are anticipated.

The huntingtin inclusion is a dynamic phase-separated compartment

Inclusions of disordered protein are a characteristic feature of most neurodegenerative diseases, including Huntington's disease. Huntington's disease is caused by expansion of a polyglutamine tract in the huntingtin protein; mutant huntingtin protein (mHtt) is unstable and accumulates in large intracellular inclusions both in affected individuals and when expressed in eukaryotic cells. Using mHtt-GFP expressed in Saccharomyces cerevisiae, we find that mHtt-GFP inclusions are dynamic, mobile, gel-like structures that concentrate mHtt together with the disaggregase Hsp104. Although inclusions may associate with the vacuolar membrane, the association is reversible and we find that inclusions of mHtt in S. cerevisiae are not taken up by the vacuole or other organelles. Instead, a pulse-chase study using photoconverted mHtt-mEos2 revealed that mHtt is directly and continuously removed from the inclusion body. In addition to mobile inclusions, we also imaged and tracked the movements of small particles of mHtt-GFP and determine that they move randomly. These observations suggest that inclusions may grow through the collision and coalescence of small aggregative particles.

Atomistic simulations of the effects of polyglutamine chain length and solvent quality on conformational equilibria and spontaneous homodimerization

Aggregation of expanded polyglutamine tracts is associated with nine different neurodegenerative diseases, including Huntington's disease. Experiments and computer simulations have demonstrated that monomeric forms of polyglutamine molecules sample heterogeneous sets of collapsed structures in water. The current work focuses on a mechanistic characterization of polyglutamine homodimerization as a function of chain length and temperature. These studies were carried out using molecular simulations based on a recently developed continuum solvation model that was designed for studying conformational and binding equilibria of intrinsically disordered molecules such as polyglutamine systems. The main results are as follows: Polyglutamine molecules form disordered, collapsed globules in aqueous solution. These molecules spontaneously associate at conditions approaching those of typical in vitro experiments for chains of length N>/=15. The spontaneity of these homotypic associations increases with increasing chain length and decreases with increasing temperature. Similar and generic driving forces govern both collapse and spontaneous homodimerization of polyglutamine in aqueous milieus. Collapse and dimerization maximize self-interactions and reduce the interface between polyglutamine molecules and the surrounding solvent. Other than these generic considerations, there do not appear to be any specific structural requirements for either chain collapse or chain dimerization; that is, both collapse and dimerization are nonspecific in that disordered globules form disordered dimers. In fact, it is shown that the driving force for intermolecular associations is governed by spontaneous conformational fluctuations within monomeric polyglutamine. These results suggest that polyglutamine aggregation is unlikely to follow a homogeneous nucleation mechanism with the monomer as the critical nucleus. Instead, the results support the formation of disordered, non-beta-sheet-like soluble molten oligomers as early intermediates--a proposal that is congruent with recent experimental data.

Cellular localization and development of neuronal intranuclear inclusions in striatal and cortical neurons in R6/2 transgenic mice

The cellular localization and development of neuronal intranuclear inclusions (NIIs) in cortex and striatum of R6/2 HD transgenic mice were studied to ascertain the relationship of NIIs to symptom formation in these mice and gain clues regarding the possible relationship of NII formation to neuropathology in Huntington's disease (HD). All NIIs observed in R6/2 mice were ubiquitinated, and no evidence was observed for a contribution to them from wild-type huntingtin; they were first observed in cortex and striatum at 3.5 weeks of age. In cortex, NIIs increased rapidly in size and prevalence after their appearance. Generally, cortical projection neurons developed NIIs more rapidly than cortical interneurons containing calbindin or parvalbumin. Few cortical somatostatinergic interneurons, however, formed NIIs. In striatum, calbindinergic projection neurons and parvalbuminergic interneurons rapidly formed NIIs, but they formed more gradually in cholinergic interneurons, and few somatostatinergic interneurons developed NIIs. Striatal NIIs tended to be smaller than those in cortex. The early accumulation of NIIs in cortex and striatum in R6/2 mice is consistent with the early appearance of motor and learning abnormalities in these mice, and the eventual pervasiveness of NIIs at ages at which severe abnormalities are evident is consistent with their contribution to a neuronal dysfunction underlying the abnormalities. That cortex develops larger NIIs than striatum, however, is inconsistent with the preferential loss of striatal neurons in HD but is consistent with recent evidence of early morphological abnormalities in cortical neurons in HD. That calbindinergic and parvalbuminergic striatal neurons develop large NIIs is consistent with a contribution of nuclear aggregate formation to their high degree of vulnerability in HD.