Cellular senescence or its equivalence is induced by treatment of cells with an appropriate inducer of senescence in various cell types. Mild restriction of cytoplasmic protein synthesis prevented induction of all aspects of cellular senescence in normal and tumor-derived human cells. It allowed the cells to continuously grow with no sign of senescent features in the presence of various inducers. It also delayed replicative senescence in normal human fibroblasts. Moreover, it allowed for growth of the cells that had entered a senescent state. When adult worms of the nematode C. elegans were grown under protein-restricted conditions, their average and maximal lifespans were significantly extended. These results suggest that accumulation of cytoplasmic proteins due to imbalance in macromolecule synthesis is a fundamental cause of cellular senescence.

Gravitaxis of swimming microorganisms has so far been explained largely in terms of the physical properties of the microorganisms that are assumed not to have any speculative mechanisms of gravity sensation. However gravity-induced sensory input and the subsequent modulation of locomotor activity in Paramecium has been suggested by precise analyses of swimming velocity as a function of swimming direction with respect to gravity. Paramecium appears to modulate its propulsive effort depending on the swimming direction by increasing the propulsive speed in upward and decreasing it in downward directions. This response termed as gravikinesis was obtained from the averaged measurement on a large number of the cell. In the present study we aimed to confirm the results on the basis of the measurement on the swimming of a single cell. Velocities of upward and downward swimming was measured from each single cell, and the velocity of sinking was measured on the same cell which was immobilized by Ni 2+. We confirmed the gravikinesis from the analysis on the swimming of single cells. In addition, we found that in the hyper-density medium containing Percoll cells showed opposite gravikinesis to that in the hypo-density medium. These results may suggest the mechanosensitivity of the cell membrane as a physiological sensor underlying the gravity-dependent response in Paramecium.

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