Role of nucleolar stress in the progression of neurodegeneration in models of Huntington´s disease.

• Applicant: Dr. Rosanna Parlato
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 253776045

• Final Report Year: 2018

Final Report Abstract

Despite its known genetic origin caused by an expanded trinucleotide CAG repeat in the Huntingtin gene, our understanding of the complex pathomechanisms underlying Huntington’s disease (HD) is still incomplete. The mutant Huntingtin (mHTT) protein affects several cellular functions hindering the identification of the primary pathogenic event. Impaired transcription of ribosomal DNA (rDNA) genes in the nucleolus – a major non-membrane bound organelle in the nucleus - represents an emerging mechanism in several neurodegenerative diseases including HD. rRNA synthesis is tightly linked to the cell wellbeing and it is finely tuned by cellular stress conditions. On the other hand, mutant proteins and RNAs may interfere with rRNA synthesis in the nucleolus. The interdependence between nucleolar dysfunction and accumulation of mHTT nuclear inclusions in HD progression remained unexplored. This project aimed at identifying: I) the role of inhibition of rRNA synthesis - a condition known as nucleolar stress - in the progression of HD and II) nucleolar stress-associated genes altering the disease course. Having as a long-term aim the development of therapeutic approaches targeting either directly or indirectly the response to nucleolar stress, this project consisted of two main tasks: i) the analysis of early cellular and molecular alterations triggered by increased nucleolar stress in HD models and ii) the identification of modulators of striatal neurodegeneration in HD models linked to dysregulation of nucleolar function. Our major findings are that the induction of nucleolar stress in dopaminoceptive neurons accelerates the onset of HD motor phenotypes in mouse models providing first evidence that nucleolar stress may modulate the effects of mHTT toxicity on disease progression. mHTT in turn differentially affects nucleolar function and integrity in a stage-specific fashion, altering the distribution of specific nucleolar proteins, such as NPM1, that depends on the state of mHTT nuclear inclusions. Our results also indicate that although rDNA transcription is unaffected in models of early disease stage, changes in nucleolar protein distribution may play a role in steering the degenerative process. Here we also show that nucleolar stress may affect the integrity of other nuclear organelles and disease-related pathways, such as RNA splicing, proteostasis, DNA repair and that some of these signatures are shared by HD models. Indeed by different experimental approaches we could show that protein synthesis is reduced in the presence of mHTT. Stimulation of rDNA transcription in mHTT-expressing cells provided first evidence that this approach might be toxic, suggesting that enhancing rRNA synthesis is fatal for these cells. In the meantime, these results also open new questions about the role played by increased rRNA transcription at early stages and by specific nucleolar proteins that appear differentially affected by mHTT. As one follow-up for this project, we currently address the possible application of the identified cellular and molecular changes for monitoring disease progression in other tissues, for example in human patients fibroblasts and skeletal muscle biopsies. The basic idea is to label nucleolar markers by immunofluorescence, and to use confocal microscopy followed by the semiquantitative analysis of the signal number, area and intensity. Although as expected, targeting nucleolar function for therapeutic applications appears challenging, the analysis of nucleolar function and integrity might represent a fundamental read-out for therapeutic efficacy of approaches lowering the level of mHTT.

Publications

• Role of nucleolar dysfunction in neurodegenerative disorders: a game of genes? AIMS Molecular Science, 2015; 2(3): 211-224
  R. Parlato and H. Bierhoff
  
• Journal of Neurology Neurosurgery & Psychiatry 2016; 87(Suppl 1):A16.1-A16
  R. Parlato, A. Soenmez, S.T. Spieth, C. Litke, R. Mustafa, G. Kreiner, H. Bierhoff, F. Tuorto, J.R. Naranjo, B. Liss
  
• Genetic mutations linked to Parkinson's disease differentially control nucleolar activity in presymptomatic mouse models. Dis Model Mech, 2017; 10(5):633-643
  V. Evsyukov, A. Domanskyi, H. Bierhoff, S. Gispert, R. Mustafa, F. Schlaudraff, B. Liss, R. Parlato
  
• Stage and cell-specific changes of nucleolar activity and integrity are associated with progression of Huntington’s disease. Acta Physiologica, 2017; 219 (52) Supplement S711
  A. Soenmez, S.T. Spieth, H. Bierhoff, R. Mustafa, J Koch, B. Liss, R. Parlato
  
• C9orf72-associated neurodegeneration in ALS-FTD: breaking new ground in ribosomal RNA and nucleolar dysfunction. Cell Tissue Res, 2018; 373(2):351-360
  D. Herrmann, R. Parlato