Soft probe particle tracking microrheology using membraneless organelles to study viscoelasticity of nucleolar...

Tracking cellular viscoelastic properties is critical for identifying early signs of pathogenesis. We introduce soft probe particle tracking microrheology (S-PTM), which leverages the intrinsic motion of membraneless organelles instead of exogenous rigid probes. Focusing on the nucleolus, S-PTM uses nested subcompartments as probes to characterize local mechanics. Unlike traditional microrheology, which employs foreign particles in nonspecific regions, S-PTM enables viscoelastic measurements of specific organelles through natural compartmentalization. Probe selection follows strict criteria: near-spherical shape, strong fluorescence, full encapsulation within the target phase, and structural integrity. Minor deviations from sphericity do not bias estimates, but pronounced asphericity or shape loss significantly alters the inferred properties. We validate S-PTM using single and double oil-water emulsions solely as benchmarks and then apply it to HeLa Kyoto cells to quantify the viscoelastic responses of nucleolar subcompartments-the dense fibrillar component and granular component-a capability unattainable with foreign-particle PTM. We extract storage modulus (G'), loss modulus (G″), and phase angle, revealing that nucleolar subcompartments behave as viscoelastic media, consistent with their crowded molecular architecture. Distinct mechanics between compartments align with hypotheses about nucleolar organization. S-PTM offers a facile, expression-controlled approach for probing subcellular mechanics and holds promise for broader applications in biophysical and synthetic systems.