Prof. Dr. Thorsten Hoppe

Research Area: Protein Homeostasis in Aging & Disease

Website: www.hoppelab.uni-koeln.de

1. Research Background:

Cellular differentiation, developmental processes, and environmental factors challenge the integrity of the proteome in every eukaryotic cell. The maintenance of protein homeostasis, or proteostasis, involves the degradation of misfolded and damaged proteins, and is essential for cellular function, organismal growth, and ultimately viability. Sustaining protein quality control (PQC) is not only a long-term challenge for individual cells but also for entire organisms, since damaged proteins accumulate with stress and aging. It is commonly thought that age-related impairment of PQC affects general proteostasis networks, causing enhanced aggregation of misfolded proteins that can be toxic for cells and shortens organismal lifespan. Not all tissues are equally susceptible to the toxicity of protein aggregates, suggesting tissue-specific differences in proteostasis pathways. In humans, aberrant protein aggregation is often associated with neurodegeneration in age-dependent disorders such as Alzheimer’s and Parkinson’s diseases.

The ubiquitin/proteasome system (UPS) is a major proteolytic route functioning in a cellular network that helps to maintain the proteome during stress and aging. Degradation of damaged proteins is mediated by the 26S proteasome upon attachment of ubiquitin proteins. Another proteolytic system supporting proteostasis is the autophagy-lysosome pathway that degrades proteins inside activated autophagosomes. An age-related impairment of either of these systems causes enhanced protein aggregation and affects lifespan, suggesting functional overlap and cooperation between UPS and autophagy in stress and aging. Despite the progress made in searching for key substrates that are destined for degradation, the major challenge in the field is to understand how these proteolytic systems are mechanistically coordinated to overcome age-related proteotoxicity. The ultimate goal of our research is to assemble a global picture of stress-induced proteolytic networks critical for aging of multicellular organisms. We address both cellular and tissue-specific regulation of protein degradation pathways using the powerful genetic model of Caenorhabditis elegans.

2. Research questions addresses by the group:

Protein homeostasis or proteostasis is achieved via a conserved network of quality control pathways that support the generation of correctly folded proteins (biogenesis), prevent proteins from misfolding (conformational maintenance), and remove potentially harmful protein species (degradation). Unfortunately, the proteostasis network has a limited capacity and its impairment causes protein aggregation that deteriorates both cellular and organismal viability. Recent studies identified cell-nonautonomous regulation of proteotoxic stress 2 response, suggesting the existence of intricately balanced proteostasis networks important for integration and maintenance of the organismal proteome. Our research particularly aims to understand the dynamic regulation of proteolytic pathways that integrate environmental and physiological changes. The detailed analysis of both cellular and tissue-related coordination of proteostasis will allow us to assemble a global picture of conserved protein degradation networks important to safeguard the organismal proteome in health and disease. Our lab aims to unravel the interplay between proteolytic networks at the molecular, cellular, and organismal level and to define adaptation mechanisms that ensure its integrity in response to environmental stress conditions or inherited, disease-associated mutations. The central research program is focussed on the following points:

  • Identification of proteolytic pathways important for stress resistance and longevity
  • Analysis of cell-type specific differences between proteolytic networks
  • Identification of stress response mechanisms for proteome maintenance
  • Addressing the systemic coordination between tissue-related proteolytic nodes
  • Defining important pathomechanisms underlying disease-related dysfunction

3. Possible projects:

The assembly and maintenance of myofilaments require a tightly balanced proteostasis network. One key player important for myosin organization and muscle thick filament formation in health and disease is the HSP90 co-chaperone UNC-45. The integrity of sarcomeric structures is permanently challenged upon muscle growth and mechanical stress. In response to eccentric exercise or damage to the myofiber, UNC-45 together with the chaperone HSP90 shuttles between the Z-disk and the myosin containing A-band of the muscle sarcomere. However, little is known about the coordination of protein homeostasis pathways upon mechanical stress. Therefore, the long-term objective of this project is to understand how the balance between protein folding and degradation networks is coordinated with myosin assembly and muscle integrity. To this end, we will combine genetic and biochemical approaches to study the conserved function of UNC-45 in myosin assembly and examine how this function is modulated during mechanical stress. Specifically, we plan to use targeted screening strategies to uncover mechanosensory proteins, chaperones, UPS and autophagy components that are required for muscle function. The conserved regulation of proteostasis networks will be studied in Caenorhabditis elegans, C2C12 mouse myoblasts, and human skeletal muscles. Finally, we will address the remodeling of the UNC-45 folding machinery under mechanical stress. A combination of genetic, biochemical and in-vivo imaging techniques will allow us to examine stress-induced changes of protein folding and degradation pathways. The proposed project will have broad implications for the understanding of myosin assembly, human myopathies and proteostasis mechanisms in general.

Objectives

The myosin assembly co-chaperone UNC-45 is essential for muscle function in health and disease. However, little is known about how the regulation of UNC-45 activity is coordinated within folding and degradation networks to maintain cellular proteostasis upon mechanical stress conditions and physical exercise. The main goal of our proposed research is to understand how the balance between protein folding and degradation is coordinated with muscle assembly, maintenance, and repair of myofilaments. We will specifically focus on the following aspects:

  1. the UNC-45 interaction network in vivo,
  2. the mechanistic regulation of UNC-45 in myosin organization,
  3. the stress-related modulation of UNC-45 regulatory networks.

Given that the organization and arrangement of sarcomere proteins are highly conserved, we believe that the research proposed in this grant will have broad implications for the understanding of molecular mechanisms protecting myofilaments against mechanical stress and proteostasis mechanisms in general.

4. Applied Methods and model organisms:

Our interdisciplinary project will combine state-of-the-art biochemical, cell biological, genetic, proteomic, and bioinformatic approaches to define the molecular mechanisms underlying proteolytic dysfunction under distinct metabolic and pathological conditions. The main model organism used is the nematode Caenorhabditis elegans, which allows an ideal combination of genetic, biochemical and in vivo imaging techniques to examine the dynamic cross talk between folding and degradation networks. Over the last years we established different degradation assays that allow to follow protein turnover in vivo. These assays were successfully used in genetic screens; subsequent whole genome sequencing/mapping techniques helped to find for example a new crosstalk between mitochondrial metabolism and cytosolic protein degradadtion pathways (Segref et al., 2014). In most projects we were able to tranfer the findings made in nematodes into human cell culture models, which are often related to protein aggregation disease.

5. Desirable skills and qualifications:

We are seeking a highly motivated PhD student to join our enthusiastic and collaborative group. Successful applicants should have a solid background in molecular biology and experience in cell biology, genetics, or biochemistry. Candidates should have demonstrated outstanding performance through their undergraduate studies. Besides creativity, a strong ability for problem solving through analytical thinking combined with an enthusiasm for scientific research is highly desirable. Additionally, we expect good communication skills, fluent English and the ability for teamwork.

6. References:

  1. Tawo, R., Pokrzywa, W., Kevei, E., Akyuz, M.E., Balaji, V., Adrian, S., Höhfeld, J., and Hoppe, T. (2017). The ubiquitin ligase CHIP integrates proteostasis and aging by regulation of insulin receptor turnover. Cell 169, 470-482.
  2. Ackermann, L., Schell, M., Pokrzywa, W., Kevei, E., Gartner, A., Schumacher, B., and Hoppe, T. (2016). E4 ligase specific ubiquitylation hubs coordinate DNA double strand break repair and apoptosis. at Struct. Mol. Biol. 23, 995-1002.
  3. Frumkin, A., Dror, S., Pokrzywa, W., Bar-Lavan, Y., Karady, I., Hoppe T., and Ben-Zvi A. (2014). Challenging muscle homeostasis uncovers novel chaperone interactions in C. elegans. Front. Mol. Bio. 6, 1:21.
  4. Segref, A., Kevei, E., Pokrzywa, W., Schmeisser, K., Mansfeld, J., Livnat-Levanon, N., Ensenauer, R., Glickman, M.H., Ristow, M., and Hoppe, T. (2014). Pathogenesis of human mitochondrial diseases is modulated by reduced activity of the ubiquitin/proteasome system. Cell Metab. 19, 642-652.
  5. Gazda, L., Pokrzywa, W., Hellerschmied, D., Lowe, T., Forne, I., Mueller-Planitz, F., Hoppe, T#., and Clausen, T. (2013) The myosin chaperone UNC-45 is organized in tandem modules to support myofilament formation in C. elegans. Cell 152, 183-195.
  6. Segref, A., Torres, S., and Hoppe, T. (2011). A Screenable in vivo Assay to Study Proteostasis Networks in Caenorhabditis elegans. Genetics. 187, 1235-40.
  7. Kim, J., Löwe, T., and Hoppe, T. (2008). Protein quality control gets muscle into shape. Trends Cell Biol. 18, 264-272.
  8. Janiesch, P.C., Kim, J., Mouysset, J., Barikbin, R., Lochmüller, H., Cassata, G., Krause, S., and Hoppe, T. (2007) The ubiquitin-selective chaperone CDC-48/p97 links myosin assembly to human myopathy. Nat. Cell. Biol. 9, 379-390.
  9. Hoppe, T., Cassata, G., Barral, J.M., Springer, W., Hutagalung, A.H., Epstein, H.F., and Baumeister, R. (2004). Regulation of the myosin-directed chaperone UNC-45 by a novel E3/E4-multiubiquitylation complex in C. elegans. Cell 118, 337-349.
  10. Kim, J., and Hoppe, T. (2006). The ubiquitin/proteasome system and muscle development. Protein Degradation 3, published by WILEY-VCH, 21-39.