Sure BioArtMED 2024-06-14 14:30 四川
撰文 | Sure
Huntington's disease (HD) is an inherited neurodegenerative disease caused by a deficiency in a single gene, Huntingtin (HTT), which is characterised by loss of motor control and decreased cognitive and emotional stability, often manifested by involuntary dance movements, hence the name Huntington's disease [1]. The disease is inherited dominantly, with an abnormally expanded specific region of the HTT gene (CAG repeat), which encodes a ployQ chain of variable length. When this polyQ chain is longer than 35 repeats, it causes abnormal protein function, which in turn damages brain cells [2]. Longer polyQ chains tend to form oligomers, aggregates, and inclusion bodies, the formation of structures that are thought to be a source of neurotoxicity [3]. Another hypothesis regarding the pathogenesis of HD is that CAG repeats may influence the progression of the disease through toxic effects at the RNA level [4]. Although the mutant mHTT protein is known to produce aggregation, the link between aggregation and neurotoxicity remains unclear.
Recently, Judith Frydman's group from Stanford University published a research paper Polyglutamine-mediated ribotoxicity disrupts proteostasis and stress responses in Huntington's disease in Nature Cell Biology. In this study, the authors explored the regulatory mechanisms of HTT protein translation and aggregation, and found that the CAG-expanding mutant HTT protein leads to premature termination of translation through ribosomal toxicity, thereby releasing mHTT protein that is prone to aggregation and further disrupting intracellular protein homeostasis and stress function. These findings not only deepen scientists' understanding of the pathogenesis of HD, but also provide potential intervention options for its treatment.
In the 5' non-coding region of human and mouse HTT mRNA, there is a conserved upstream open reading frame (uORF), which is located before the major ORF and can play a regulatory role in the translation of mRNA into protein. When the authors analyzed the ribo-seq data, they found that both uORF and major ORFs had ribosomal priming, and when treated with cycloheximide (CHX) to block the elongation of translation, the ribosomes occupied the entire region of the uORF but did not extend to the major ORF region, suggesting that uORF inhibited the translation of the HTT gene in vivo by reducing the priming of major ORF translation. However, most of the mRNA translation efficiency decreased under the stress response induced by thapsigargin (Tg) treatment, but HTT and another Atf4 gene, which also contains uORF, showed enhanced mRNA translation under stress conditions. These results suggest that the distinct translational behavior of uORF-containing mRNAs in different settings may have an important impact on the pathogenesis and disease progression of HD. Further analysis showed that uORF controlled the aggregation of HTT protein by regulating the initiation of translation, which could delay the formation of inclusion bodies and reduce the generation of mHTT fragments that tend to aggregate. In addition, the use of the translation initiation inhibitor 4EGI-1 can further reduce the production of these neurotoxic fragments and improve symptoms in animal models.
Next, the authors investigated the role of polyQ sequences in HD disease development. They used a viral vector to express, label, and detect the nascent HTT strand in cells, and found that the expansion of the polyQ strand increases the risk of ribosomal collision and premature termination, leading to the release of aggregation-prone mHTT fragments. When translating mHTT proteins with longer polyQ chains, more translational quality control (ROC) and protein quality control (PQC) factors, especially those associated with the proteasome's clearance of mHTT, are recruited. In contrast to this is a significant reduction in the translation elongation factor eIF5A in translating mHTT proteins, and its interaction with mHTT may lead to aggregation and affect neurological function. At the same time, changes in eIF5A were associated with worsening of symptoms of disease as HD mouse models aged, suggesting that ribosomal collision and associated degradation of ubiquitination may be key factors in HD neurodegeneration. More importantly, the authors also found that mHTT interacts with eIF5A and blocks its function, altering translational elongation on hundreds of mRNAs, which in turn disrupts protein homeostasis, a process that creates a vicious cycle of protein homeostasis dysfunction and therefore may exacerbate HD symptoms.
To confirm the possibility of the authors' finding, the researchers then explored how cells in HD respond to endoplasmic reticulum stress and misfolded protein responses caused by polyQ expansion. The study found that HD cells were deficient in dealing with the stress of a protein fold error, which caused phosphorylation of the translational initiator eIF2a, and the resulting integrated stress response. In particular, the reduction of eIF5A caused by mHTT hindered the effective translation of protective factors, and the expression of protective factors in HD cells under endoplasmic reticulum stress could be increased to a certain extent by supplementing eIF5A. The authors believe that by reducing the loss of eIF5A or increasing its activity, it could be a potential strategy for the treatment of HD.
Finally, the authors suggest that if mHTT makes cells more sensitive to stress, then HD cells will be particularly sensitive to drugs that increase ribosomal collisions by slowing down translational elongation. However, these cells respond weakly to drugs that slow down translational initiation, so reducing the rate of translational initiation can reduce ribosomal collisions and premature termination. These results suggest that in HD cells, ribosomal collision and related stress responses can be potentially alleviated by regulating the balance between translation initiation and elongation, which provides a possible intervention avenue for HD treatment.
Overall, this article identified a target to reduce mHTT levels and combat its cytotoxicity, CAG expansion leads to translation termination due to ribosomal collision, releasing neurotoxic mHTT fragments, thereby insulating eIF5A and other protein quality control factors through aggregation, leading to impaired protein homeostasis in neuronal cells and inhibiting the induction of protective stress responses. These findings will help guide the treatment of HD disease and provide new intervention strategies.
Original link:
https://doi.org/10.1038/s41556-024-01414-x
Plate maker: Eleven
bibliography
1. Bates, G. P. et al. Huntington disease. Nat. Rev. Dis. Prim. 1, 15005 (2015).
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3. Jimenez-Sanchez, M., Licitra, F., Underwood, B. R. & Rubinsztein, D. C. Huntington’s disease: mechanisms of pathogenesis and therapeutic strategies. Cold Spring Harb. Perspect. Med. 7, a024240 (2017).
4. Nalavade, R., Griesche, N., Ryan, D. P., Hildebrand, S. & Krauß, S. Mechanisms of RNA-induced toxicity in CAG repeat disorders. Cell Death Dis. 4, e752 (2013).