Molecular crowding and RNA synergize to promote phase-separation, microtubule interaction, and seeding of Tau condensates (2022)
Figure 4

Figure 4. Tubulin and RNA compete for condensed Tau.

Tubulin co-condensation with Tau. Addition of tubulin-Alexa647 (5 μM, 10:1 unlabeled:labeled tubulin) to a Tau-Dy488 solution (25 μM) induces Tau condensation. Addition of tubulin to Tau:PEG, Tau:RNA (polyA-Cy3), and Tau:PEG:RNA condensates leads to partitioning of tubulin into existing condensates and the occurrence of additional small Tau:tubulin condensates. Condensates with high tubulin-HiLyte647 intensity have low polyA-Cy3 intensity, and vice versa. In Tau:PEG:polyA condensates, individual condensates host RNA- and tubulin-rich domains (arrow heads) at homogeneous Tau-Dy488 intensity. Scale bars=20 μm.

Average intensity of RNA (top graph) normalized to Tau intensity in individual condensates. Addition of tubulin reduces the average amount of RNA in condensates. Intensity of tubulin (middle graph) normalized to Tau intensity in individual condensates. The presence of RNA increases the amount of tubulin in the condensates. Data shown as mean±SD. Data +/- tubulin and +/- polyA were compared by two-tailed Student's t-tests. Tubulin and RNA intensity correlate negatively on an individual condensate level (bottom graph). Line represent linear regression to all data. N=3 experiments. *P<0.05, **P<0.01, ***P<0.001, ****P<0.0001.

Condensate size (are on surface) relative to tubulin (yellow data points) and RNA (pink data points) intensity. Smaller (new) condensates are high in tubulin, larger (old) condensates have more RNA. Lines represent linear regressions to tubulin (yellow) and RNA (pink) data.

MT polymerization out of Tau:PEG and Tau:PEG:RNA condensates. Addition of tubulin-Alexa647 (5 μM, 10:1 unlabeled:labeled tubulin) and GTP to Tau-Dy488 (25 μM) diluted in BRB80 polymerization buffer induces MT polymerization. Images taken 30 min after addition of tubulin show thin partially curved MT bundles in Tau:PEG system, and thick bundles attached to persisting Tau:RNA condensates in the Tau:PEG:RNA system. Scale bar=5 μm.

Cross-sectional intensity plots (line plots) of MT bundles differ between Tau:PEG (yellow/red line plots) and Tau:PEG:RNA (blue/green line plots) systems. Four line plots at random positions are shown per condition.

Time course of MT polymerization out of Tau:PEG:RNA condensates. Scale bar=20 μm. Box refers to panel G.

Close-up of Tau condensed phase connecting RNA condensate with MT bundles. Arrow indicates position of individual MT bundle positioned on top of RNA and underneath Tau in condensed phase. Scale bar=5 μm.

Nocodazole treatment (120 μM for 30 min) of MTs formed from Tau:PEG:RNA and Tau:PEG condensates. MT formed in absence of Tau shown as control. Graphs show surface coverage (%) of fibrillar MT structures before and after nocodazole treatment. N=3 experiments, 3 images analyzed per experiment and condition. Data shown as mean±SD. Two-tailed Student's t-test. *P<0.05, **P<0.01, ***P<0.001, ****P<0.0001. Scale bar=20 μm.

Attachment of Tau condensates to negatively-charged (glutamate treated) glass surfaces. Tau:RNA and Tau:PEG:RNA condensates show pronounced surface wetting, whereas Tau:PEG condensates stay in solution or show partial wetting. p-Tau:PEG condensates also wet the surfaces. Scale bars=10 μm.