Johbu Itoh, Natsuko Fujii, Saori Kohara, Masatoshi Ito, Naoto Suzuki, Eiichiro Nagata
Liquid-liquid phase separation (LLPS) plays a central role in intracellular compartmentalization and gene regulation. Although optogenetic optoDroplet systems are widely used to study LLPS in living cells, the mechanisms underlying the transition from dynamic liquid-like condensates to operationally defined gel-like assemblies remain poorly understood because intermediate assembly states are difficult to monitor in situ. Here, we developed a confocal laser microspectroscopy-based approach to characterize the assembly dynamics of the fused sarcomas (FUS)n-mCherry-CRY2 optoDroplet system in living NIH3T3 cells. Quantitative fluorescence analysis enabled the identification and discrimination of three distinct assembly states: monomeric, dimer/oligomer intermediate, and gel-like. Passive micro-rheology, fluorescence recovery after photobleaching (FRAP), and automated morphological analyses further demonstrated progressive reductions in molecular mobility and increasing structural rigidity during condensate maturation. Three-dimensional spatial analysis revealed that mature condensates exhibit a distinct core-shell organization consisting of an operationally defined gel-like core, a dimer/oligomer-rich intermediate layer, and an outer monomer-rich boundary layer with liquid-like properties. Upon cessation of blue-light stimulation, the outer layer rapidly dissolved, whereas the central gel-like core remained intact, indicating the acquisition of physical irreversibility. These findings establish fluorescence microspectroscopy as a quantitative approach for resolving molecular assembly states during condensate maturation and demonstrate that gelation proceeds from the condensate center toward the periphery. This framework provides new insights into pathological liquid-to-gel phase transitions associated with neurodegenerative diseases.