Huntington’s Disease & Worms…What?

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Yup, you read that right. Huntington’s Disease and worms, specifically microscopic worms such as caenorhabditis elegans (c.elegans). In my first year of graduate school, I had to come up with a theoretical project on a neurodegenerative disease of my choice. I’d thought it would be cool if I posted parts of that paper, specifically the background information and the significance of the research. One of my favourite aspects of science (other than chemistry and physics) is neuroscience, and whenever I get the chance I enjoy researching topics within the field. I hope you guys find this, at least, slightly interesting. At the bottom, I’ve included the references listed. If you have any questions, feel free to ask, and I can try to send you the correct sources :).


Characterising Cytotoxic Forms of Mutant Huntingtin (mHTT) in Huntington’s Disease

Background: Huntington’s disease (HD) is a fatal autosomal dominant inherited neurodegenerative disease first described by George Huntington in 18721,2. HD, also known as chorea, is characterized by the following: abrupt, uncontrollable, rapid movement (choreatic movement); changes in behavior and intellectual abilities; and dementia1. HD is rare, affecting 1 out of 10,000 persons, and is caused by CAG repeats in the huntingtin gene (HTT), located on chromosome 4, which encodes for the HTT protein1,2. The specific function of the HTT protein at the biochemical level remains unknown, however, studies have been done that illustrate huntingtin plays a significant role in, but not limited to, nervous system development, influence of brain-derived neurotrophic factor (BDNF) production and transport, and cell adhesion1,3. HTT protein is expressed in various of locations throughout the body; but, it is heavily concentrated in the brain, specifically the neurons1. Because there is a high propensity of HTT protein concentrated in the brain, this could be the reason as to why there is such a high level of neuronal cell death in HD, in particular, GABAergic neurons, thus leading to motor deficits.

In HD, the HTT gene is mutated, which causes atypical amounts of CAG repeats—a strong indicator of HD; this leads to an abnormal polyQ (Glutamine) expansion in the protein, which can lead to cell toxicity, disruption of cellular processes, and, ultimately, cell death1–3. The severity of the disease is dependent of the number of CAG repeats in the gene. The mutated HTT (mHTT) gene contains a disordered segment known as the HTT exon1 fragment, which contains three domains: a proline rich domain (PRD), a polyQ expansion, and an N-terminal domain called HTTNT, or N171. The HTT exon1 fragment is cleaved from the mHTT full length protein in one of two ways: a missplicing event during the transcription and processing of the HTT gene or during post-translational modification due to proteolytic cleavage1. These fragments accumulate in the cell producing aggregates that disrupt cellular processes and are dependent on polyQ length expansions and nucleation1,4–6.

How the HTT gene is mutated or how mHTT protein affects cellular processes remains a mystery. Over the years, there have been hypotheses suggested that could possibly explain the mechanism of HD pathogenesis. One hypothesis suggested is that cytoplasmic aggregates (inclusion bodies) of mHTT induce cytotoxicity and is a potential cause of HD. However, evidence7 has shown that these inclusion bodies may not cause HD but are rather a protective feature created in HD to slow cell death. This led to the hypothesis that there are certain conformations of mHTT that induce cytotoxicity within the cell5,7. This hypothesis, conversely, has had mixed results and is a controversial subject in HD research.  There have been research studies that allude to toxic forms of mHTT. In these studies, researchers have shown that there are certain molecules that alter aggregation and oligomeric forms of mHTT such as TRiC—a chaperonin—which showed to decrease toxicity and modify aggregation8.  On the other hand, there are studies that contradict this hypothesis9,10. Klein, et al., 2013 argues that, in great accumulation and/or with slower kinetics, non-mutant HTT protein can form aggregates and induce cytotoxicity9.

Significance: The conflicting nature of these studies question whether or not if there is a cytotoxic form of mHTT that causes and/or exacerbates HD. Nevertheless, in order to answer the question if there is (are) a cytotoxic conformation(s) of mHTT, the possible conformations of mHTT need to be ascertained, isolated, and observed inducing cytotoxic activity.  If proven true, these conformations can be used as a target for therapeutic treatment and even as a biomarker for severity of cell death—a momentous advancement in the treatment of HD. If proven false, researchers can focus their attention on other mechanisms and molecular moieties. Establishing if there are toxic forms of mHTT will open doors in HD research and research up until this point has suggest that there is a toxic form of mHTT,which is why we propose the following:

Hypothesis: There are (monomeric and/or oligomeric) conformations of the mutant HTT (mHTT) protein that induce cytotoxicity, which leads to oligomerization and aggregation of polyQ repeat proteins (e.g. mHTT), formation of inclusion bodies, neuronal dysfunction, and cell death. By isolating these toxic conformations, we can confirm that there are conformations of mHTT that induce cytotoxicity and can be targeted in therapeutic treatments as a method to slow and/or stop the progression of HD.

Why worms? Worms in neurological disorders is a practically unexplored area–not much research within the neuroscience field has explored the concept of using microscopic worms as a tool to study the effects of neurological disorders. The use of worm models is normally seen in genetics/sex determination studies. The importance of worms in neuroscience research is profound and would open up avenues of research for scientists to explore and to not be limited by cell culture and human models. Though few, there has been some Huntington’s Disease research that has used worms, specifically C. elegans, as their model.11

References:

  • Bates, G. P. et al. Huntington disease. Nat. Rev. Dis. Prim. 15005 (2015). doi:10.1038/nrdp.2015.5
  • Finkbeiner, S. Huntington’s disease. Cold Spring Harb. Perspect. Biol. 3, (2011).
  • Zuccato, C., Valenza, M. & Cattaneo, E. Molecular Mechanisms and Potential Therapeutical Targets in Huntington’s Disease.
  • Brignull, H. R., Morley, J. F., Garcia, S. M. & Morimoto, R. I. Modeling Polyglutamine Pathogenesis in C. elegans. Methods Enzymol. 412, 256–282 (2006).
  • Arrasate, M. & Finkbeiner, S. Protein aggregates in Huntington’s disease. Experimental Neurology 238, 1–11 (2012).
  • Chen, S., Ferrone, F. A. & Wetzel, R. Huntington’s disease age-of-onset linked to polyglutamine aggregation nucleation. PNAS 99, 11884–11889 (2002).
  • Arrasate, M., Mitra, S., Schweitzer, E. S., Segal, M. R. & Finkbeiner, S. Inclusion body formation reduces levels of mutant huntingtin and the risk of neuronal death. Nature 431, 805–810 (2004).
  • Behrends, C. et al. Chaperonin TRiC Promotes the Assembly of polyQ Expansion Proteins into Nontoxic Oligomers. Mol. Cell 23, 887–897 (2006).
  • Klein, F. A. C. et al. Linear and extended: a common polyglutamine conformation recognized by the three antibodies MW1, 1C2 and 3B5H10. Hum. Mol. Genet. 22, 4215–4223 (2013).
  • Wetzel, R. Physical chemistry of polyglutamine: Intriguing tales of a monotonous sequence. Journal of Molecular Biology (2012). doi:10.1016/j.jmb.2012.01.030
  • Cornaglia, M. et al. Automated longitudinal monitoring of in vivo protein aggregation in neurodegenerative disease C. elegans models. Mol. Neurodegener. 11, 17 (2016)

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