The extracellular signal-regulated kinase (ERK) plays a central role in the signaling cascades of cell growth. Here, we show that stochastic ERK activity pulses regulate cell proliferation rates in a cell density-dependent manner. A fluorescence resonance energy transfer (FRET) biosensor revealed that stochastic ERK activity pulses fired spontaneously or propagated from adjacent cells. Frequency, but not amplitude, of ERK activity pulses exhibited a bell-shaped response to the cell density and correlated with cell proliferation rates. Consistently, synthetic ERK activity pulses generated by a light-switchable CRaf protein accelerated cell proliferation. A mathematical model clarified that 80% and 20% of ERK activity pulses are generated by the noise and cell-to-cell propagation, respectively. Finally, RNA sequencing analysis of cells subjected to the synthetic ERK activity pulses suggested the involvement of serum responsive factor (SRF) transcription factors in the gene expression driven by the ERK activity pulses.

Phenotypic heterogeneity of cancer cells is caused not only by genetic and epigenetic alterations but also by stochastic variation of intracellular signaling molecules. Using cells that stably express Fö rster resonance energy transfer (FRET) biosensors, we show here a correlation between a temporal fluctuation in the activity of Rac1 and the invasive properties of C6 glioma cells. By using longterm time-lapse imaging, we found that Rac1 activity in C6 glioma cells fluctuated over a timescale that was substantially longer than that of the replication cycle. Because the relative level of Rac1 activity in each cell was unaffected by a suspension-adhesion procedure, we were able to sort C6 glioma cells according to the levels of Rac1 activity, yielding Among the 14 genes that were most associated with the term 'membrane' (membrane-related genes) in Rac1 high cells, we identified four genes that were associated with glioma invasion and Rac1 activity by using siRNA knockdown experiments. Among the transcription factors upregulated in Rac1 high cells, Egr2 was found to positively regulate expression of the four membranerelated invasion-associated genes. The identified signaling network might cause the fluctuations in Rac1 activity and the heterogeneity in the invasive capacity of glioma cells.

There is still a strong requirement of effective therapeutics for overcoming the COVID-19 pandemic. Up to 68% of the total amino acid residues encoded in the SARS-CoV-2 genome have been currently covered by the structural data. The recent activities in drug discovery against COVID-19 are reviewed in this article with emphasizing a protein structure-based approaches.

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