Regulatory role of calcium mobilization in retinal neurogenesis

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Intracellular calcium is a regulatory signal for cell proliferation. To reveal calcium signal dynamics during cell cycle, we applied calcium fluorescence imaging to the retinal neuroepithelium of chick embryo (embryonic day 3, E3, and E6), where the soma changes its position during the cell cycle. Purinoceptors were stimulated by the application of adenosine triphosphate (ATP) to cause calcium release from calcium stores, since the purinoceptor activation promotes DNA synthesis. The ATP-induced calcium rises occurred in the nucleoplasm of cells at around S phase as revealed by confocal fluorescence microscopy in the E3 retina. The calcium rise declined in M-phase cells and postmitotic cells (retinal ganglion cells). These results were confirmed in the vertical slice preparation of the E6 retina. The soma of S-phase cell is located in the inner layer of the retinal neuroepithelium and issues an outer process, which extends to the ventricular surface. Fluorescent probes for endoplasmic reticulum (ER) showed that the ERs in the outer process and the nuclear envelope or peri-nuclear ER formed the calcium store. Depletion of the calcium store induced capacitative calcium entry, which caused calcium rises in the terminal of outer process and soma. These results suggest that the calcium signaling to nucleus via the store-operated system including the ERs in the outer process is crucial for the cell cycle progression in the retinal neuroepithelium. Spontaneous calcium oscillation was also observed in the nucleoplasm of S-phase cells. The mechanism of calcium oscillation and the functional significance of the polarized structure of retinal neuroepithelium for calcium signaling will be studied.