Metabolite supply in oocytes of Nereis virens: role of nucleosides

Abstract

Gamete development in Nereis virens and other nereidid polychaetes is organized in a simple way. Follicular tissues are absent and the germ cells develop floating freely in the coelomic fluid, which thus serves as the vehicle for the supply of substances required for oocyte growth. This overview focusses on the role of exogenous purine nucleosides for the growing oocytes. Eleocytes, a coelomic cell type which is proliferated in large amounts at the beginning of sexual maturation, supply purine nucleosides to support nucleic acid synthesis in the oocytes. Eleocytes can store large amounts of purine nucleotides (up to 50 µ mol ml−1 cell vol.) in the form of AMP and ADP. During oogenesis, these nucleotide stores are degraded. A transient increase in the intracellular concentration of inosine found in eleocytes at the time of nucleotide degradation and a release of inosine and guanosine measured in cultivated eleocytes suggests that the stored adenine nucleotides are catabolized to inosine (INO) and guanosine (GUO) which are exported to the coelomic fluid. Oocytes of all stages can take up 14C-INO and -GUO by an ATP dependent, saturable uptake system. The uptake rates are highest at the beginning of the rapid growth phase; this event corresponds in the eleocytes with the time of nucleotide breakdown, nucleoside production and nucleoside release. In uptake experiments with oocytes using physiological concentrations of 14C-INO, the specific radioactivity of the intracellular INO pool reached that of INO in the incubation medium after 48h suggesting that the INO pool in the oocytes would be fed mainly via the coelomic fluid under in vivo conditions. Both 14C-INO and -GUO are converted to adenine and guanine nucleotides in oocytes with a further incorporation into the nucleic acid fractions. Utilization of INO for nucleic acid synthesis shows two maxima during oocyte growth: in the early phase of slow growth, both INO and GUO are almost exclusively incorporated into the RNA fraction. In the subsequent, rapid growth phase, both nucleosides are incorporated into both RNA and DNA fractions. During the latter phase, INO concentrations in the oocytes are at a minimum reflecting an increased consumption for nucleic acid synthesis. Metabolism of exogenous INO and GUO by oocytes is relatively slow under natural temperature conditions. At 12 oC, up to 25% of the totally incorporated 14C-nucleosides were found in the nucleic acid fractions after 24h. Our data provide evidence for the utilization of eleocyte-derived exogenous purine nucleosides by the growing oocytes of Nereis virens. However, pyrimidine nucleosides are equally required for nucleic acid synthesis but are not released by eleocytes. Oocytes store large concentrations of the pyrimidine nucleotide precursors cytosine and cytidine. However, it is not known at present whether these compounds are taken up from exogenous sources or are synthesized within the oocytes.