Endoplasmic reticulum (ER) calcium as modulator of neuronal protein synthesis
Studies performed with non-neuronal cells demonstrated that emptying ER calcium stores led to a blocking of the initiation process of protein synthesis (PS), a phenomenon also observed in neuronal tissue after transient ischemia. To study this mechanism, we used hippocampal cell cultures, kept in vitro for 9 days. ER calcium stores were depleted by incubating cultures for 30 min in medium containing l u M of the ER Ca2+-ATPase inhibitor thapsigargin (Tg). PS was evaluated with labelled leucine. Cytoplasmic calcium activity was measured by Fura 2 fluorescence microscopy. For electron microscopic (EM) detection of the state of ribosomal aggregation cells were cultured on plastic foil, fixed and processed for EM by standard techniques. Upon T g treatment PS was reduced to 15% of control. Buffering cytosolic calcium with BAPTA reduced PS to 60%. In BAPTA pre-loaded cells application of Tg led to a decrease of PS to 10%. Tg treatment for 45 min produced only a small transient increase in the E340/E380 fluorescence ratio, whereas 100 mM glutamate elicited a massive long-lasting increase in this ratio. After 30 min Tg incubation polyribosomes appeared to be completely disaggregated. Following a recovery period of 4 hours typical features of apoptosis (like membrane blebbing and condensation of chromatin) were present in some neurons. The results indicate that Tg treatment inhibited PS almost completely, most likely due to the depletion of ER calcium stores and not due to the small increase in cytosolic calcium activity since the inhibition could not be reversed by chelating cytoplasmic calcium. Thus, in neuronal cells a disturbance of ER calcium homeostasis induces a disaggregation of polyribosomes, inhibition of protein synthesis and morphological changes indicative of apoptosis, i.e. a pattern similar to that evoked by transient cerebral ischemia.
Doutheil, J; Gissel, C; Paschen, W
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