The Emerging Landscape of Natural Small-molecule Therapeutics for Huntington's Disease

Huntington's disease (HD) is a rare and fatal neurodegenerative disorder with no diseasemodifying therapeutics. HD is characterized by extensive neuronal loss and is caused by the inherited expansion of the huntingtin (HTT) gene that encodes a toxic mutant HTT (mHTT) protein having expanded polyglutamine (polyQ) residues. Current HD therapeutics only offer symptomatic relief. In fact, Food and Drug Administration (FDA) approved two synthetic small-molecule VMAT2 inhibitors, tetrabenazine (1) and deutetrabenazine (2), for managing HD chorea and various other diseases in clinical trials. Therefore, the landscape of drug discovery programs for HD is evolving to discover disease- modifying HD therapeutics. Likewise, numerous natural products are being evaluated at different stages of clinical development and have shown the potential to ameliorate HD pathology. The inherent anti-inflammatory and antioxidant properties of natural products mitigate the mHTT-induced oxidative stress and neuroinflammation, improve mitochondrial functions, and augment the anti-apoptotic and pro-autophagic mechanisms for increased survival of neurons in HD. In this review, we have discussed HD pathogenesis and summarized the anti-HD clinical and pre-clinical natural products, focusing on their therapeutic effects and neuroprotective mechanism/s.

Molecular Strategies to Target Protein Aggregation in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by the aggregation of the mutant huntingtin (mHTT) protein in nerve cells. mHTT self-aggregates to form soluble oligomers and insoluble fibrils, which interfere in a number of key cellular functions. This leads to cell quiescence and ultimately cell death. There are currently still no treatments available for HD, but approaches targeting the HTT levels offer systematic, mechanism-driven routes towards curing HD and other neurodegenerative diseases. This review summarizes the current state of knowledge of the mRNA targeting approaches such as antisense oligonucleotides and RNAi system; and the novel methods targeting mHTT and aggregates for degradation via the ubiquitin proteasome or the autophagy-lysosomal systems. These methods include the proteolysis-targeting chimera, Trim-Away, autophagosome-tethering compound, autophagy-targeting chimera, lysosome-targeting chimera and approach targeting mHTT for chaperone-mediated autophagy. These molecular strategies provide a knowledge-based approach to target HD and other neurodegenerative diseases at the origin.

An Expanded Polyproline Domain Maintains Mutant Huntingtin Soluble in vivo and During Aging

Huntington's disease is a dominantly inherited neurodegenerative disorder caused by the expansion of a CAG repeat, encoding for the amino acid glutamine (Q), present in the first exon of the protein huntingtin. Over the threshold of Q39 HTT exon 1 (HTTEx1) tends to misfold and aggregate into large intracellular structures, but whether these end-stage aggregates or their on-pathway intermediates are responsible for cytotoxicity is still debated. HTTEx1 can be separated into three domains: an N-terminal 17 amino acid region, the polyglutamine (polyQ) expansion and a C-terminal proline rich domain (PRD). Alongside the expanded polyQ, these flanking domains influence the aggregation propensity of HTTEx1: with the N17 initiating and promoting aggregation, and the PRD modulating it. In this study we focus on the first 11 amino acids of the PRD, a stretch of pure prolines, which are an evolutionary recent addition to the expanding polyQ region. We hypothesize that this proline region is expanding alongside the polyQ to counteract its ability to misfold and cause toxicity, and that expanding this proline region would be overall beneficial. We generated HTTEx1 mutants lacking both flanking domains singularly, missing the first 11 prolines of the PRD, or with this stretch of prolines expanded. We then followed their aggregation landscape in vitro with a battery of biochemical assays, and in vivo in novel models of C. elegans expressing the HTTEx1 mutants pan-neuronally. Employing fluorescence lifetime imaging we could observe the aggregation propensity of all HTTEx1 mutants during aging and correlate this with toxicity via various phenotypic assays. We found that the presence of an expanded proline stretch is beneficial in maintaining HTTEx1 soluble over time, regardless of polyQ length. However, the expanded prolines were only advantageous in promoting the survival and fitness of an organism carrying a pathogenic stretch of Q48 but were extremely deleterious to the nematode expressing a physiological stretch of Q23. Our results reveal the unique importance of the prolines which have and still are evolving alongside expanding glutamines to promote the function of HTTEx1 and avoid pathology.

Structural insights into the aggregation mechanism of huntingtin exon 1 protein fragment with different polyQ-lengths

Huntington disease is a neurodegenerative disorder caused by the expansion of polyglutamine (polyQ) at the N-terminal of the huntingtin exon 1 protein. The detailed structure and the mechanism behind this aggregation remain unclear and it is assumed that the polyQ undergoes a conformational transition to the β-sheet structure when it aggregates. Investigating the misfolding of polyQ facilitates the determination of the molecular mechanism of aggregation and can potentially help in developing a novel approach to inhibit polyQ aggregation. Moreover, the flanking sequences of the polyQ region play a vital role in structural changes and the aggregation mechanism. We performed all-atom molecular dynamics simulations to gain structural insights into the aggregation mechanism using eight different models with glutamine repeat lengths Q₂₇ , Q₂₇ P₁₁ , Q₃₄ , Q₃₅ , Q₃₆ , Q₄₀ , Q₅₀ , and Q₅₀ P₁₁ . In the models without flanking polyPs, we noticed that the transformation of a random coil to β-sheet occurs when the number of Q increases. We also found that the flanking polyPs prevent aggregation by decreasing the probability of forming a β-sheet structure. When polyQ length increases, the 17 N-terminal flanking residues are more likely to adopt a β-sheet conformation from α-helix and coil. From our simulations, we suggest that at least 34 glutamines are required for initiating aggregation and 40 residues length is critical for the aggregation of huntingtin exon 1 protein for disease onset. This study provides structural insights into misfolding and the role of flanking sequences in huntingtin aggregation which will further help in developing therapeutic strategies for Huntington's disease.