Ca pulsation in BY-2 cells and evidence for control of mechanosensory Ca-selective channels by the plasmalemmal reticulum.

A previously unknown cytoskeletal structure, now named the plasmalemmal reticulum (Gens et al. 2000, Protoplasma 212, 115-134), was found in cultured BY-2 tobacco cells during a search for a force-focusing mechanism that might enhance signal transduction by the cells' mechanosensory Ca-selective cation channels (MCaCs). This polyhedral structure, which links cell wall, plasma membrane, and internal cytoplasm, prominently contains arabinogalactan protein (AGP). To check for reticulum-promoted Ca elevation, the AGP-binding reagent (β-d-glucosyl) Yariv phenylglycoside has been applied to BY-2 cells expressing a free cameleon Ca reporter. Ca elevation was substantial and prolonged. Moreover it occurred in the nucleus as well as the cytoplasm. Cells treated with non-binding mannosyl Yariv reagent could not be discriminated from untreated controls or those treated with carrier solution alone. Supply of the MCaC inhibiter Gd just before treatment with Yariv reagent prevented Ca rise. These data strongly support the hypothesis that the plasmalemmal reticulum controls MCaC activity. The massive inward spread of Ca suggested that entry of the ion through the channels initiated a wave of release from the ER, and YCX in the ER showed Ca levels consistent with this premise. Cytosolic and nuclear Ca often pulsed in control cells in near synchrony and at rates ranging from zero to five cycles per ∼20-min recording. (Pulsation was over-ridden by the applied amounts of glucosyl Yariv compound.) Suggestively but very crudely, oscillation rate was assessed as possibly correlating with stage of cell cycle. Because cell Ca was lowered and pulsation was eliminated by Gd, MCaCs appear to participate in these endogenous fluctuations. The extent to which pulsing plays regulatory roles in relatively undifferentiated types of cells should be evaluated.