Nicking of rat spermatid and spermatozoa DNA: possible involvement of DNA topoisomerase II.

Chromatin of rat elongating spermatids, steps 12-13, is distinguished by the replacement of histones with transition proteins and the presence of nicks within its DNA which are formed by an endogenous nuclease, possibly DNA topoisomerase II (topo II). Using an affinity-purified anti-topo II antibody, protein bands of approximately 161 and approximately 137 kDa were detected on immunoblots of pachytene spermatocytes and elongating spermatids, respectively. In cryosections, topo II was localized to meiotic chromosomes of pachytene spermatocytes and to nuclei of elongating spermatids. Extracts of isolated testicular nuclei and sonication-resistant spermatid nuclei (steps 12-19) demonstrated topo II activity as determined by the decatenation of kinetoplast DNA. The potential relationship between nucleoprotein changes during spermatogenesis and the formation of nicks was also examined. Heterogeneous testicular and sonication-resistant spermatid nuclei were treated with 0.8 mM protamine, followed by nick translation in the absence of DNase I. In both cases, there was a dramatic decrease in DNA polymerase I-dependent label incorporation. To determine whether or not endogenous nicks were present in mature sperm, but were inaccessible due to protamine-DNA interactions, epididymal sperm were extracted with high salt-dithiothreitol, followed by nick translation in the absence or presence of DNase I. Extracted sperm nuclei did not nick translate in the absence of DNase I; however, incorporation increased with increasing concentrations of DNase I, indicating that endogenous nicks were repaired prior to the completion of spermatogenesis. These and previously published results suggest that topo II in elongating spermatids may be involved in the DNA alterations that take place during spermatogenesis, including changes in DNA topography, repair, and loop formation, and may serve as a component of the nuclear matrix. The temporal appearance and disappearance of endogenous nicks may reflect the changes that elongating spermatid DNA undergoes as a consequence of alterations in nucleoprotein composition to establish the condensed state of the mature spermatozoon.