Amyloid β (Aβ) oligomers play a critical role in the pathology of Alzheimer's disease. Recently, we reported that a conformation‐restricted Aβ42 with an intramolecular disulfide bond through cysteine residues at positions 17/28 formed stable oligomers with potent cytotoxicity. To further optimize this compound as a toxic conformer model, we synthesized three analogues with a combination of cysteine and homocysteine at positions 17/28. The analogues with Cys‐Cys, Cys‐homoCys, or homoCys‐Cys, but not the homoCys‐homoCys analogue, exhibited potent cytotoxicity against SH‐SY5Y and THP‐1 cells even at 10 nM. In contrast, the cytotoxicity of conformation‐restricted analogues at positions 16/29 or 18/27 was significantly weaker than that of wild‐type Aβ42. Furthermore, thioflavin‐T assay, non‐denaturing gel electrophoresis, and morphological studies suggested that the majority of these conformation‐restricted analogues exists in an oligomeric state in cell culture medium, indicating that the toxic conformation of Aβ42, rather than the oligomeric state, is essential to induce cytotoxicity.

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Photobiomodulation has analgesic effects via inhibition of nerve activity, but few reports have examined the effects on the spinal dorsal horn, the entry point for nociceptive information in the central nervous system. In this study, we evaluated the effects of laser irradiation of peripheral nerve axons, which are conduction pathways for nociceptive stimuli, on the neuronal firing in lamina II of the spinal dorsal horn of a rat evoked by mechanical stimulation with von Frey filaments (vFF). In order to record neuronal firing, electrodes were inserted into lamina II of the exposed rat spinal dorsal horn. The exposed sciatic nerve axons were irradiated with an 808 nm laser. The 26.0 g vFF-evoked firing frequency was inhibited from 5 min after laser irradiation and persisted for 3 h. Sham irradiation did not alter the firing frequency. Laser irradiation selectively inhibited 15.0 and 26.0 g vFF-evoked firing, which corresponded to nociceptive stimuli. Histopathological evaluation revealed no damage to the sciatic nerve due to laser irradiation. These results indicate that neuronal firing is inhibited in lamina II of the spinal dorsal horn, suggesting that laser irradiation inhibits Aδ and/or C fibers that conduct nociceptive stimuli.

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that requires further pathological elucidation to establish effective treatment strategies. We previously showed that amyloid β (Aβ) toxic conformer with a turn at positions 22–23 is essential for forming highly toxic oligomers. In the present study, we evaluated phenotypic changes with aging in AD model AppNL-P-F/NL-P-F (NL-P-F) mice with Swedish mutation (NL), Iberian mutation (F), and mutation (P) overproducing E22P-Aβ, a mimic of toxic conformer utilizing the knock-in technique. Furthermore, the role of the toxic conformer in AD pathology was investigated. NL-P-F mice produced soluble toxic conformers from an early age. They showed impaired synaptic plasticity, glial cell activation, and cognitive decline, followed by the accumulation of Aβ plaques and tau hyperphosphorylation. In addition, the protein expression of hypoxia-inducible factor (HIF)-1α was increased, and gene expression of HIF-3α was decreased in NL-P-F mice. HIF dysregulation due to the production of soluble toxic conformers may be involved in AD pathology in NL-P-F mice. This study could reveal the role of a highly toxic Aβ on AD pathogenesis, thereby contributing to the development of a novel therapeutic strategy targeting the toxic conformer.

Since amyloid β (Aβ) oligomers are more cytotoxic than fibrils, various dimer models have been synthesized. We focused on the C-terminal region that could form a hydrophobic core in the aggregation process and identified a toxic conformer-restricted dimer model (E22P,G38DAP-Aβ40 dimer) with an L,L-2,6-diaminopimelic acid linker (n = 3) at position 38, which exhibited moderate cytotoxicity. We synthesized four additional linkers (n = 2, 4, 5, 7) to determine the most appropriate distance between the two Aβ40 monomers for a toxic dimer model. Each di-Fmoc-protected two-valent amino acid was synthesized from a corresponding dialdehyde or cycloalkene followed by ozonolysis, using a Horner−Wadsworth−Emmons reaction and asymmetric hydrogenation. Then, the corresponding Aβ40 dimer models with these linkers at position 38 were synthesized using the solid-phase Fmoc strategy. Their cytotoxicity toward SH-SY5Y cells suggested that the shorter the linker length, the stronger the cytotoxicity. Particularly, the E22P,G38DAA-Aβ40 dimer (n = 2) formed protofibrillar aggregates and exhibited the highest cytotoxicity, equivalent to E22P-Aβ42, the most cytotoxic analogue of Aβ42. Ion mobility-mass spectrometry (IM-MS) measurement indicated that all dimer models except the E22P,G38DAA-Aβ40 dimer existed as stable oligomers (12−24-mer). NativePAGE analysis supported the IM-MS data, but larger oligomers (30−150-mer) were also detected after a 24 h incubation. Moreover, E22P,G38DAA-Aβ40, E22P,G38DAP-Aβ40, and E22P,G38DAZ-Aβ40 (n = 5) dimers suppressed long-term potentiation (LTP). Overall, the ability to form fibrils with cross β-sheet structures was key to achieving cytotoxicity, and forming stable oligomers less than 150-mer did not correlate with cytotoxicity and LTP suppression.