The role of Hsp42 in proteostasis-metabolism interplay and cellular longevity
by Alice Lippi
Date of Examination:2024-06-28
Date of issue:2025-01-23
Advisor:Dr. Anita Krisko
Referee:Dr. Anita Krisko
Referee:Prof. Dr. Gerhard Braus
Persistent Address:
http://dx.doi.org/10.53846/goediss-10952
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Abstract
English
Aging is a natural phenomenon characterized by a progressive decline despite complex pathways of maintenance and repair. Over time, cells experience profound changes in cell morphology as well as alterations in gene expression and shifts in metabolic pathways, all of which accompanied by a decline in proteostasis. The loss of proteostasis in aging results in the inability of proteins to achieve and maintain their native three-dimensional structure and perform their functions efficiently. To counteract this, cells have evolved mechanisms to promote the removal of damaged proteins through degradation and nucleation and growth of protein aggregates. The functions of protein refolding, targeting to degradation and aggregate nucleation are all performed by molecular chaperones. During aging, as well as stressful conditions, cells activate stress response pathways to cope with the increase of misfolded proteins and promote survival. As part of this response, cells prioritize the translation of stress response genes over housekeeping genes, resulting in the formation of cytoplasmic stress granules (SGs). Additionally, nutrient sensing mechanisms play a pivotal role in orchestrating the intricate interplay between metabolism and cell lifespan. As cells progress through their lifespan, metabolic activity undergoes profound shifts, particularly notable in glucose sensing. Despite the critical role of proteostasis on multiple levels of cellular homeostasis, the interplay between proteostasis and other cellular pathways in aging remains poorly understood. Using budding yeast S. cerevisiae as a model system, we aim to investigate how changes in proteostasis affect protein condensation, modulate metabolic pathway activity and extend lifespan. Our study reveals a previously unknown role of Hsp42 in SG dynamics during the response to glucose starvation. We found that SG formation and dissolution are stepwise processes in wild- type cells, involving fusion and fission events, respectively. However, in the absence of Hsp42, these events are disrupted, leading to a failure in SG fusion and the inability of the disaggregation machinery (Hsp70-Hsp104) to colocalize with Pab1, a well-known SG marker. Notably, this phenotype is attributed to the N-terminal domain of Hsp42. Our results suggest that Hsp42 modulates the fluidity and organization of SGs, influencing the sorting and arrangement of molecules within the granules both in live cells and in vitro, highlighting a new crucial role of Hsp42. Moreover, our results show that the absence of Hsp42 leads to a 42% extension of yeast replicative lifespan (RLS) accompanied by metabolic reprogramming. This includes a switch to respiratory metabolism, despite the abundance of glucose, and therefore an increase in oxygen consumption accompanied by a decline in glucose uptake. The most prominent metabolic outcome downstream of Hsp42 deletion is the deactivation of the TOR1 pathway, which itself has a major impact on lifespan extension. While the absence of Hsp42 displayed lethal phenotype in combination of constitutively active Tor1 kinase, other experiments have shown that the switch to respiration is essential for the observed RLS extension. These results illustrate dynamic communication between proteostasis, metabolism and lifespan driven by Hsp42 deficiency.
Keywords: aging; proteostasis; metabolism; yeast; protein aggregation; hsp42; hsp; chaperone; lifespan
