Peptides have recently garnered attention as middle-molecular-weight drugs with the characteristics of small molecules and macromolecules. Lysine-specific demethylase 1 (LSD1) is a potential therapeutic target for lung cancer, neuroblastoma, and leukemia, and some peptide-based LSD1 inhibitors designed based on the N-terminus of SNAIL1, a member of the SNAIL/SCRATCH family of transcription factors, have been reported. The N-terminus of SNAIL1 peptide acts as a cap of the catalytic site of LSD1, inhibiting interactions with LSD1. However, the structure-activity relationship (SAR) of these inhibitors is not yet fully understood. Therefore, in the present study, we aimed to uncover the SAR and to identify novel SNAIL1 peptide-based LSD1 inhibitors. We synthesized peptide inhibitor candidates based on truncating the N-terminus of SNAIL1 or substituting its amino acid residues. In the truncation study, we found that SNAIL1 1-16 (2), which was composed of 16 residues, strongly inhibited LSD1. Furthermore, we investigated the SAR at residues-3 and -5 from the N-terminus and found that peptides 2j and 2k, in which leucine 5 of the parent peptide is substituted with unnatural amino acids, cyclohexylalanine and norleucine, respectively, strongly inhibited LSD1. This result suggests that the hydrophobic interaction between the inhibitor peptides and LSD1 affects the LSD1-inhibitory activity. We believe that this SAR information provides a basis for the development of more potent LSD1 inhibitors.

Amyloid proteins are widely studied, both for their unusual biophysical properties and their association with disorders such as Alzheimer\'s and Parkinson\'s disease. Fluorescence-based methods using site-specifically labeled proteins can provide information on the details of their structural dynamics and their roles in specific biological processes. Here, we describe the application of different labeling methods and novel fluorescent probe strategies to the study of amyloid proteins, both for in vitro biophysical experiments and for in vivo imaging. These labeling tools can be elegantly used to answer important questions on the function and pathology of amyloid proteins.