Effect on memory of acute administration of naturally secreted fibrils and synthetic amyloid-beta peptides in an invertebrate model.

Amyloid beta peptide (Abeta) is considered one of the main agents of Alzheimer's disease pathogenesis. Recently, it has been proposed that memory deficits are caused by different stages of Abeta aggregation, particularly by oligomers. In addition, although memory impairment was found after Abeta administration in rodents and chicks, the nature of the memory deficits induced in invertebrates by acute administration of mammalian Abeta peptides is not well understood. Previously, we reported the amnesic effect of acute pre-training administration of naturally formed fibrils (NF) in crab memory. Here we evaluate the effect of NF and synthetic Abeta peptides administration at different times before and after training in this well characterized invertebrate memory model, the context-signal memory of the crab Chasmagnathus. We found a clear amnesic effect at very low doses of naturally Abeta NF only when administered immediately pre- and post-training, but not 24 h and 18 h before or 6h after training. Activation of ERK/MAPK (a protein kinase required for memory formation in this model) 60 min after administration was found. In contrast, neither JNK/SAPK nor NF-kappaB transcription factor were activated. Furthermore, synthetic Abeta1-42 and Abetapy3-42 administration induced amnesia when used after a protocol for fibrillation but not after a protocol for oligomerization. On the contrary, no amnestic effect was found when fibrillated Abeta1-40 and Abetapy11-42 peptides were used. Thus, Abeta1-42 and Abetapy3-42 peptides impaired memory and the effects were only found when highly aggregated peptides, which may include fibrils, protofibrils and oligomers, were administered. These temporally- and signaling-specific effects suggest that Abeta impairs memory by inducing transient physiological, rather than permanent neuropathological, alterations of the brain and this effect is achieved through generalized ERK activation.

Neuronal fibrillogenesis: amyloid fibrils from primary neuronal cultures impair long-term memory in the crab Chasmagnathus.

Amyloid beta protein (Abeta) fibrillogenesis is considered one of the crucial steps of Alzheimer's disease (AD) pathogenesis. The effect of endogenous neuronal amyloid fibrils on memory processes is unknown. To investigate this issue, we first characterised the Abeta fibrillar aggregates secreted by cerebellar granule cells and then we evaluated the effect of neuronal fibrils on an invertebrate model of memory. An increase of fibril formation, assessed by Thioflavin T (ThT) fluorescence, was observed in the conditioned medium of apoptotic neurons during 48 h of the apoptotic process. Moreover, the depolarisation-stimulated secretion of cerebellar granule cells contains monomers of endogenous Abeta, which undergo cell-free fibrillogenesis over several days of incubation. The pattern of single endogenous fibrils, examined by electron microscopy, was similar to that of synthetic Abeta while a tighter and more complex interfibrillar organization was observed in endogenous fibrils. The biological effect of neuronal fibrils was studied in a long-term memory (LTM) paradigm, namely the context-signal learning of the crab Chasmagnathus. Pre-training injection of neuronal fibril extract (protein concentration, 1 microg/ml) induced amnesia in a dose-dependent manner. On the contrary, no effect on retention was observed with the administration of two orders higher doses (100 microg/ml) of synthetic Abeta1-40. These results indicate that only naturally secreted fibrils, but not synthetic Abeta, clearly interfere with memory process.