Fluorescence anisotropy uncovers changes in protein packing with inclusion growth in a cellular model of polyglutamine aggregation.

The aggregation of polyglutamine-rich proteins is closely linked with numerous neurodegenerative disorders. In pathological and cellular models, the appearance of protein-rich inclusions in cells acts as a read out of protein aggregation. The precise organization of aggregated protein in these inclusions and their mode of growth are still poorly understood. Here, fluorescence anisotropy-based measurements have been used to probe protein packing across inclusions of varying brightness, formed by an monomeric enhanced green fluorescent protein (mEGFP)-tagged polyglutamine model peptide in cells. High-resolution, confocal-based steady-state anisotropy measurements report a large depolarization, consistent with extensive homo-Förster (fluorescence) resonance energy transfer (FRET) between the sequestered mEGFP-tagged protein molecules. An inverse correlation of fluorescence anisotropy with intensity is seen across inclusions, which becomes emphasized when the observed fluorescence anisotropy values of inclusions are corrected for the fluorescence contribution of the diffusible protein, present within and around smaller inclusions. Homo-FRET becomes enhanced as inclusion size increases. This enhancement is confirmed by two-photon excitation-based time-resolved fluorescence anisotropy decay measurements, which also suggest that the mEGFP-tagged protein molecules are arranged in multiple ways within inclusions. Bright inclusions display faster FRET rates with a larger number of mEGFP moieties participating in homo-FRET than faint inclusions do. These results are consistent with a model in which the protein is more closely packed in the brighter inclusions. In such a possible mechanism, the higher packing density of protein molecules in brighter inclusions would suggest that inclusion growth could involve an intermolecular compaction event within the inclusion, as more monomers and aggregates are recruited into the growing inclusion.