CELLULAR AND SUBCELLULAR CALCIUM ION HOMEOSTASIS

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This project focuses on the mechanism whereby cells achieve homeostasis of Ca2+ ion concentration, both within the cytosol and other cellular compartments, and allow changes in Ca2+ in response of hormones and neurotransmitters. We are currently focussing on the role of deranged Ca2+ homeostasis in the process of cell death. This year we have investigated the effect of the overexpression of the bc12 oncogene in the Jurkat T cell line upon regulation of cytosol [Ca2+] and the thapsigargin-sensitive pool of endoplasmic reticulum Ca2+. In response to serum-starvation, the bc12-transfected cells survive longer and maintain a larger thapsigargin-releasable pool of Ca2+ than do the control cells. There may be a cause-and-effect relationship between these two parameters. We have also asked the question of whether killing of cells by exposure to visible light in the presence of porphyrins involves the peripheral benzodiazepine receptor in the mitochondrial membrane, the opening of the mitochondrial "megachannel" and the consequent release of a pulse of Ca2+ into the cytosol. In studies with mitochondrial isolated from heart and liver, exposure to porphyrins and light was found to be extremely effective in inhibiting mitochondrial Ca2+ uptake and in causing release of accumulated Ca2+. However, the effect was not blocked by cyclosporin, which inhibits the opening of the megachannel, and did involve uncoupling, as shown by fluorescence of the membrane potential-sensitive dye JC-1. Thus, the induction of damage by photodynamic therapy is broader in mechanism than the opening of this channel alone. These studies on Ca2+ in cell death are geared towards an understanding of the basic biology of this process, with an eventual goal of designing interventions to prevent unwanted cell death in fixed, post-mitotic tissues during aging.