Structural characterization of protein states involved in Huntington’s disease by integrative approaches – SPIN-HD

Huntington’s disease (HD) is a rare and lethal neurodegenerative pathology caused by mutation of the gene huntingtin that results in a protein with an expanded tract of glutamines (PolyGln). Individuals with glutamine tracts larger than 35 in the N-terminal region of huntingtin (N-Htt) have a high propensity to present amyloidogenic fibres in neurons that are the hallmark of HD. Despite the extensive knowledge of HD at clinical level, the connection at molecular detail between huntingtin structure and the disease is poorly understood. In this project, SPIN-HD, the structural study of the relevant species of huntingtin involved in HD is proposed. There are several aspects of HD that make this molecular description difficult and SPIN-HD will try to overcome previously found limitations with original biophysical tools. These tools will be of general application to other misfolding diseases and could notably contribute to unravel their molecular origin.
The first aim of SPIN-HD is the structural characterization of N-Htt which is an Intrinsically Disordered Protein (IDP). The inherent structural plasticity of IDPs requires novel strategies accounting for the coexistence of multiple conformations in solution. Besides, structural studies of huntingtin have been severely hampered by the lack of appropriate tools to study the conformational landscape explored by low-complexity regions such as PolyGln and PolyPro tracts present in N-Htt. Here, tools to comprehensively integrate state-of-the-art NMR and SAXS data will be designed. The structural models of pathological and non-pathological huntingtin variants will unravel the structural perturbations exerted by abnormally extended PolyGln tracts.
Soluble oligomeric species found along the process of fibre formation in HD and other neurodegenerative diseases such as Alzheimer and Parkinson are considered as the putative toxic agents that interfere with multitude of pathways originating these pathologies. The second objective of SPIN-HD is the structural and kinetic description of the oligomeric species present in the amyloidogenic processes in HD. This characterization has been hampered by the intrinsic polydispersity found along the fibrillation pathway. Proper mathematical treatment of time-dependent SAXS measurements will disentangle the inherent complexity of the process and will provide insights into the structure and kinetics of the species involved in HD amyloidosis.
The effect exerted of small organic molecules known to interact with N-Htt in the formation of amyloid fibres will be evaluated combining in vitro and in vivo experiments. The compilation of these results will provide evidences on two different facets. First, it will allow the identification of the citotoxic species. Second, it will suggest compounds with the capacity to disrupt these toxic species and to stop or delay cellular damage.
Strategies designed to pursue this project are intended to be of general applicability. Thus, as a long-term objective they will be exported to other misfolding diseases such as Alzheimer’s and Parkinson’s diseases, and other PolyGln disorders.
In summary, SPIN-HD is intended to tackle the molecular origin of HD by developing appropriate tools that will extend present knowledge of disordered proteins and amyloidogenic processes. The final aim is a deeper understanding of the molecular events that are on the origin of these devastatiting pathologies.