Meiotic activation of rat pachytene spermatocytes with okadaic acid: the behaviour of synaptonemal complex components SYN1/SCP1 and COR1/SCP3

The phosphatase inhibitor okadaic acid accelerates meiotic events in rodent germ cells in culture. Isolated pachytene spermatocytes treated with okada ic acid proceed to a metaphase I arrest in a few hours as opposed to the si milar process in vivo, which requires several days. Leptotene/zygotene sper matocytes cannot be activated in this way, suggesting that okadaic acid ena bles cells to bypass a sensor of the meiotic progression, which is pachyten e specific. We monitored the chromosome behaviour accompanying the transiti on to metaphase I in rat spermatocytes with antibodies against COR1/SCP3, a component of the meiotic chromosome cores, and against the synaptic protei n, SYN1/SCP1. Okadaic acid induced a rapid synaptonemal complex dissolution and bivalent separation, followed by chromosome condensation and chiasmata formation, similar to the succession of events in untreated cells. The sim ilarity between meiosis I induced with okadaic acid and the meiosis I event s in vivo extends to the dissolution of the nuclear membrane and the disapp earance of the microtubule network at the onset of metaphase I. This cell c ulture system provides a model for the in vivo transition from pachytene to metaphase I and therefore can be used in the study of this transition at t he molecular level. The effect of okadaic acid is most likely mediated by t he activation of tyrosine kinases, as addition of genistein, a general tyro sine kinase inhibitor, completely abolishes the observed effect of okadaic acid on chromosome metabolism. The okadaic acid-induced progression to the metaphase I arrest is not affected by the inhibition of protein synthesis. However, pachytene spermatocytes incubated in the presence of protein synth esis inhibitors for 6 hours show loss of synapsis which is abnormal in that it is not accompanied by chiasmata formation. The two meiosis-specific pro teins, SYN1/SCP1 and COR1/SCP3, are efficiently phosphorylated in vitro by extracts from isolated pachytene cells. Extracts from cells that have reach ed metaphase I upon okadaic acid treatment, with concomitant displacement o f SYN1/SCP1 and COR1/SCP3 from their chromosomes, do not have this capabili ty. These data support the hypothesis that phosphorylation of SYN1/SCP1 and COR1/SCP3 targets their removal from the chromosomes and that activity of the kinases involved correlates with the presence of these two proteins on the chromosomes.