Molecular Basis of Orb2 Amyloidogenesis and Blockade of Memory Consolidation

Abstract

Amyloids are ordered protein aggregates that are typically associated with neurodegenerative diseases and cognitive impairment. By contrast, the amyloid-like state of the neuronal RNA binding protein Orb2 in Drosophila was recently implicated in memory consolidation, but it remains unclear what features of this functional amyloid-like protein give rise to such diametrically opposed behaviour. Here, using an array of biophysical, cell biological and behavioural assays we have characterized the structural features of Orb2 from the monomer to the amyloid state. Surprisingly, we find that Orb2 shares many structural traits with pathological amyloids, including the intermediate toxic oligomeric species, which can be sequestered in vivo in hetero-oligomers by pathological amyloids. However, unlike pathological amyloids, Orb2 rapidly forms amyloids and its toxic intermediates are extremely transient, indicating that kinetic parameters differentiate this functional amyloid from pathological amyloids. We also observed that a well-known anti-amyloidogenic peptide interferes with long-term memory in Drosophila. These results provide structural insights into how the amyloid-like state of the Orb2 protein can stabilize memory and be nontoxic. They also provide insight into how amyloid-based diseases may affect memory processes.

The “Jekyll and Hyde” Actions of Nucleic Acids on the Prion-like Aggregation of Proteins

“…In another study, they found that Orb2 aggregation is similar to the aggregation of other proteins related to diseases (80). For example, they found the formation of toxic amyloid oligomers.…”

“…Toxicity is typical of amyloid oligomers. As discussed above for Orb2, the toxic effect might depend on the kinetics of the formation of the intermediate and its evolution into fibrils (80). The toxicity of mutant p53 oligomers would not kill the cell (similar to Orb2) but would perturb protein homeostasis, likely resulting in the release of misfolded proteins, which could interact with proteins from different compartments in the cell or even move into other cells in the tissue.…”

Long-term memory consolidation: The role of RNA-binding proteins with prion-like domains

“…Consistent with this, the N-terminus of Aplysia CPEB (as well as related domains from Drosophila and mouse orthologs) can confer prion-like behavior to heterologous fusion proteins. [134][135][136] For example, when ApCPEB N-terminus fusions with a constitutively active Glucocorticoid Receptor (GR) are expressed in yeast cells carrying a GR-responsive, b-gal reporter, it is possible to easily read out the activity state of the GR-fusion protein from the color of colonies (white when inactive and blue when active). In this assay the fusion proteins produces metastable blue and white colonies, switch from blue (active) to white (inactive) colonies at rates consistent with prion-like conformational transition.…”

“…150 Importantly, although Orb2 oligomeric complexes share biochemical features with pathological amyloids, the proposed Orb2 amyloids appear uniquely transient in nature in contrast to classical irreversible and pathological amyloid assemblies. 135 The inherent differences in the properties of the 2 isoforms also impinge on the domains of the isoforms specifically required for maintenance of LTM. Flies lacking the Q/N domain in Orb2A but not Orb2B show severe LTM impairment.…”

The TDP‐43 N‐terminal domain structure at high resolution

“…The peptide corresponding to residues 78-102 in TDP- 43 Ca chemical shift assignments were obtained (Fig. 8A,B).…”