Biochemical and Structural Studies on Amyloid Aggregation in C. elegans

Organismal proteostasis is dependent on communication mechanisms such as stress responses. These multicellular organisms require intercellular stress responses which can act beyond the confines of individual cells to activate the proteostasis network (PN) across tissues to protect organismal health. One such mechanism is transcellular chaperone signalling (TCS). When proteostasis is compromised, misfolded proteins can accumulate, giving rise to detrimental conditions such as Alzheimer's disease, characterized by the aggregation of extracellular Aβ1-42 plaques in the human brain.
TCS can be triggered by the tissue-specific over-expression or depletion of HSP- 90, which in turn activates distinct cell-non-autonomous signalling pathways. These pathways are instrumental in safeguarding entire organisms, such as the model organism C. elegans. The means by which this tissue-to-tissue communication enhances systemic chaperone expression and its potential to defend organisms against protein misfolding diseases remain unresolved questions.
To shed light on this, C. elegans were used to investigate the mechanisms by which HSP-90 over-expression or depletion initiates TCS. This research revealed that over-expressing HSP-90 induces inter-tissue signalling from the intestine to neurons, thereby protecting against Aβ1-42 aggregation. Conversely, HSP-90 depletion in the intestine triggers TCS signalling to muscle cells, leading to the activation of HSP-70 expression, and aiding in TCS-mediated heat stress resistance and lifespan extension. The pathway identified highlights the role of
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TXT-1 and CEH-58, a PDZ-domain guanylate cyclase and a homeobox transcription factor respectively, as peptides which stimulate extracellular peptides TXT-4 and TXT-8 to serve as trans-tissue signals to increase the HSP- 90 levels in distal tissues.
To further explore Aβ1-42 aggregation in C. elegans, a workflow was developed to elucidate the in-tissue architecture of Aβ1-42 fibrils. This was achieved using advanced techniques such as cryogenic correlative light electron microscopy (cryo- CLEM) and cryogenic electron tomography (cryo-ET). Ultra-thin cryo-sections of adult C. elegans tissue were collected on cryo-EM grids, and cryo-CLEM was employed to accurately map the location of Aβ1-42 fibrils, providing precise guidance for the collection of cryo-ET data. The resulting 3D reconstructions unveiled the macromolecular features of adult C. elegans tissue and offered insights into the molecular architecture of Aβ1-42 fibrils. These fibrils were found to be organized into parallel bundles, in proximity to ribosomes and microtubules.