Polyglutamine tracts as autophagy regulators

By Yoskaly Lazo-Fernandez, PhD

Polyglutamine tracts (polyQ tracts) are long chains of glutamine amino acidspresent in the sequence of many proteins. The length of polyQ tracts within proteins varies significantly as normal alleles of specific genes usually contain different number of the cytosine-adenine-guanine (CAG) nucleotide repeats¹.

It has long been known that polyQ tracts play a role in the etiology of several inheritable neurodegenerative disorders, including spinocerebellar ataxia, and Huntington's disease². These 'polyglutamine diseases' result from the excessive elongation of a polyQ tract in a particular gene which causes the resultant protein to become toxic. The toxicity of mutated polyQ tracts has been extensively studied and several explanatory hypotheses have been proposed: aggregation of polyQ tract proteins, transcriptional dysregulation, mitochondrial dysfunction, and impairment of both the ubiquitin-proteasome and autophagy-lysosome protein degradation systems.

Western blot analysis of Htt in four different lymphoblast HD cell lines extracts each expressing ~65 (mutant) and ~20 (normal) CAG repeats. Lanes 1-4  (A), HTT mAb clone 1A771 recognizes only the expanded or long form of Htt. (B), A different Htt antibody recognizing both normal and mutant Htt.

Interestingly, the physiological function of polyQ tracts in normal proteins remained unclear until very recently.  This breakthrough has been published in a Nature paper³ that depicted very detailed and exhaustive studies performed by David Rubinsztein's group at the University of Cambridge in the UK. This group uncovered an interesting regulatory function of normal polyQ tract-containing protein ataxin 3 on autophagy.

In their experiments, Ashkenazi et al.³ explored whether the knockdown or overexpression of genes involved in polyQ diseases like ataxin 3 (related to spinocerebellar ataxia) and huntingtin (Huntington's disease) affected autophagosome formation, an essential step for proper autophagy function⁴. The knockdown of wild type ataxin 3 impaired autophagosome formation in different biological models including cultured mouse neurons and liver cells, as well as in immortalized human cultured cells. Conversely, the overexpression of ataxin 3 in the same models stimulated autophagosome formation. Most importantly, the authors identified the key component of autophagosome biogenesis that is affected by the modulation of ataxin 3 expression, beclin 1, which is a particularly important protein for the induction of autophagy after nutrient depletion^4,5. Ataxin 3 was shown to be essential for the protection of beclin 1 from polyubiquitination and degradation, which made much sense since ataxin 3 is a deubiquitinating enzyme involved in the regulation of protein homeostasis. In fact, the polyQ tracts in ataxin 3 are necessary for its binding to and deubiquitination of beclin 1. Interestingly, longer polyQ tracts diminished the deubiquitinating activity of ataxin 3 and created a much stronger binding affinity of the mutated ataxin 3 to beclin 1. Longer polyQ tracts in huntingtin and other polyQ proteins also bonded strongly to beclin 1, leading to a competitive blockade of the functional interaction between wild type ataxin 3 and beclin 1, and thus resulting in excessive beclin 1 degradation and autophagy impairment.

Overall, this new study provides insight into the physiological function of normal polyQ tracts as binding domains to beclin 1, and therefore as regulators of autophagy. Also, this insight reveals a new integrative model by which mutated polyQ tract-containing proteins may cause disease. According to this new model, mutated polyQ tracts create a stronger than normal binding to beclin 1, which then blocks ataxin 3's binding and deubiquitination of beclin 1, causing beclin 1's excessive degradation. This sequence of events results in impaired basal levels of autophagy in polyQ disease patients which could explain the progressive toxic protein accumulation and aggregation observed in their neurons. Moreover, since impaired autophagy has been implicated in many chronic diseases like cancer and diabetes, perhaps the use of modulators targeting this new mechanism of autophagy regulation may provide alternative pharmacological treatments.

Learn more about autophagy regulation

References

1. Rinaldi & Fischbeck. Pathological Mechanisms of Polyglutamine Diseases. Nature Education 8, (2015).
   
2. Fan et al. Polyglutamine (PolyQ) Diseases: Genetics to Treatments. Cell Transplantation 23, 441–458(18)
3. Ashkenazi et al. Polyglutamine tracts regulate beclin 1-dependent autophagy. Nature 545, 108–111 (2017).
4. Cohen-Kaplan, Livneh, Avni, Cohen-Rosenzweig & Ciechanover. The ubiquitin-proteasome system and autophagy: Coordinated and independent activities. The International Journal of Biochemistry & Cell Biology 79, 403–418 (2016).
5. Kang, Zeh, Lotze & Tang. The Beclin 1 network regulates autophagy and apoptosis. Cell Death & Differentiation 18, 571–580 (2011).

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Beclin 1

polyQ tract

Autophagy

Ataxin 3

Huntingtin

neurodegeneration

Ubiquitin

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