A new paper in Cell, shows that the formation of amyloid-like oligomers of a RNA-binding protein in synaptic membranes is necessary for the persistence of memories in Drosophila.
If memory is the maintenance of learning associated changes in synaptic efficacy and number, and these synaptic modifications are associated with changes in protein composition, how do they persist for years when individual protein molecules are turned over in days?
Cytoplasmic polyadenylation element-binding proteins (CPEBs) are a family of RNA-binding proteins that have been shown to regulate activity-dependent protein synthesis at synapses. A Drosophila CPEB, Orb2, has been shown to be required for the long-term persistence of memory (Keleman et al.). Previous studies on a CPEB from the sea slug Aplysia, had shown that it had prion-like properties, in that it could exist in two conformational states: a monomeric form and a self-perpetuating multimeric form with amyloid-like properties. Synaptic stimulation leads to the conversion of Aplysia CPEB into the multimeric form, and the formation of a self-sustaining synaptic mark. Blocking the activity of multimeric ApCPEB inhibited the long-term facilitation of a synaptic response (Si et al.). Based on these Aplysia studies, Majumdar et al. hypothesised that Orb2′s role in Drosophila long-term memory could also be mediated by activity-dependent formation of amyloid-like oligomers.
Purifying Orb2 from Drosophila head extracts, Majumdar et al. show that it either exists in a monomeric form, or in oligomers consisting of between two and six Orb2 molecules. Orb2 oligomers are resistant to denaturation by heat or detergents, showing that they have amyloid-like characteristics. The oligomeric form of Orb2 appears to be localised to synaptic membranes, whilst the monomeric form is mainly found in cell bodies. Stimulation of neurons involved in memory formation increased the level of oligomeric Orb2.
Majumdar et al. go on to show that there are two different isoforms of Orb2 involved in the formation of Orb2 oligomers, Orb2A and Orb2B. Although the Orb2 oligomers found in vivo were mainly composed of Orb2B, the researchers found Orb2 oligomerisation requires Orb2A. Both isoforms contain a glutamine rich unstructured prion-like domain that acts as a substrate for oligomerisation, but a short 8 amino acid N-terminal domain of Orb2A catalyses the oligomerisation reaction. Orb2A appears to regulate the formation of amyloid-like Orb2 oligomers.
The previous study (Keleman et al.) that showed that Orb2 was involved in long-term memory stabilisation had used a technique that reduced the Orb2 concentration. By generating flies that carried a single point mutation in the Orb2A N-terminal domain, specifically impairing Orb2 oligomerisation, Majumdar et al. went on to show Orb2 oligomerisation is necessary for long-term memory persistence.
This work is exciting on two different fronts. It shows a mechanism for the generation of stable synaptic marks by the formation of amyloid-like oligomers. This could be a more widespread phenomenon underlying memory formation and persistence. More broadly, this study yields insight into potential physiological roles for amyloid formation and prion-like mechanisms.
Insoluble amyloid-like aggregations are a common pathological characteristic in human neurodegenerative conditions such as Alzheimer’s disease and Parkinson’s disease. In the case of prion diseases such as Creutzfeld-Jacob disease, amyloidogenic forms of Prion-Protein propagate the disease by seeding the conformational changes to the non-pathogenic form. The authors of the paper do not call the Orb2 oligomers prions, as the mechanisms underlying oligomerisation are not understood. However, in the case of ApCPEB, multimerisation was shown to be self-sustaining, and hence prion-like.
Is there a link between the preponderance of amyloid associated neurodegenerative diseases and the role of amyloid-like oligomers in memory? It is probably unwise to extrapolate too far from these results. However, one could postulate that if this is a widespread mechanism of memory formation, amyloid associated diseases could be more likely to affect the brain. Likewise, if amyloid-like protein multimerisation has normal physiological roles in the brain, it could be a more permissive environment for amyloid formation than other tissues.
Majumdar, A., Cesario, W., White-Grindley, E., Jiang, H., Ren, F., Khan, M., Li, L., Choi, E., Kannan, K., Guo, F., Unruh, J., Slaughter, B., & Si, K. (2012). Critical Role of Amyloid-like Oligomers of Drosophila Orb2 in the Persistence of Memory Cell, 148 (3), 515-529 DOI: 10.1016/j.cell.2012.01.004
Si, K., Choi, Y., White-Grindley, E., Majumdar, A., & Kandel, E. (2010). Aplysia CPEB Can Form Prion-like Multimers in Sensory Neurons that Contribute to Long-Term Facilitation Cell, 140 (3), 421-435 DOI: 10.1016/j.cell.2010.01.008
Keleman, K., Krüttner, S., Alenius, M., & Dickson, B. (2007). Function of the Drosophila CPEB protein Orb2 in long-term courtship memory Nature Neuroscience, 10 (12), 1587-1593 DOI: 10.1038/nn1996