CONTROL MECHANISMS OF MEIOTIC CELL DIVISION IN XENOPUS LAEVIS OOCYTES (METABOLISM, CALMODULIN, CYCLIC-AMP)
Abstract
Five biochemical models describing possible events leading to the induction of meiotic cell division in Xenopus laevis oocytes were tested. Two of these models predict that a small decrease in cAMP levels acts as a second messenger to the natural inducer, progesterone. A small statistically significant decrease in cAMP content was found when steps were taken to reduce experimental variability, thus providing key support for the cAMP and protein phosphorylation-dephosphorylation models. Calmodulin levels were found not to increase during maturation and no major redistribution of calmodulin during maturation was found. The accumulation of calmodulin during oogenesis was examined as well as the absolute rates of calmodulin synthesis at different stages of oogenesis. Calmodulin was microinjected into oocytes, yet did not induce maturation as expected from previous reports. Experiments with EGTA preincubation and divalent cations, indicated that changes in the ionic milieu would allow the calmodulin injection buffer alone to induce maturation. Also, divalent cations in general, and not specifically calcium, induced maturation. Several additional arguments were presented against a role for calcium and calmodulin in triggering of oocyte maturation. Changes in energy metabolism and pyridine nucleotide levels were compared with changes reported to occur in sea urchin eggs during fertilization. Like sea urchin eggs, Xenopus oocytes showed an early increase in the rate of oxygen consumption upon stimulation. However, maturing Xenopus oocytes did not show similar increases in glucose-6-phosphate levels, glucose-6-phosphate dehydrogenase activity, oxidation of glucose to CO(,2), or a major conversion of NAD to NADP. These important differences are discussed as well as the potential role of energy metabolism and pyridine nucleotides in oocyte maturation. The internal pH of Xenopus oocytes was found to elevate during maturation by a mechanism involving an acid efflux, but pH changes did not appear to play a causal role in oocyte maturation. In conclusion, a new and more intricate model of oocyte maturation was presented involving phospholipid metabolism as a key step in the induction process.
Degree
Ph.D.
Subject Area
Biology
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