Kinetoplast morphology and segregation pattern as a marker for cell cycle progression in Leishmania donovani

Trypanosomatids are typified by uniquely configured mitochondrial DNA - the kinetoplast. The replication timing of kinetoplast DNA (kDNA) is closely linked to nuclear S phase, but nuclear and kinetoplast compartments display staggered timing of segregation, post-replication. Kinetoplast division is completed before nuclear division in Trypanosoma species while nuclear division is completed first in Crithidia species. Leishmania donovani is the causative agent of visceral leishmaniasis, a form of leishmanial infection that is often fatal. Cell cycle related studies in Leishmania are hampered by difficulties in synchronizing these cells. This report examines the replication/segregation pattern and morphology of the kinetoplast in L. donovani with the aim of determining if these traits can be used to assign cell cycle stage to individual cells. By labeling replicating cells with bromodeoxyuridine after synchronization with hydroxyurea, we find that although both nuclear and kDNA initiate replication in early S phase, nuclear division precedes kinetoplast segregation in 80% of the cells. The kinetoplast is roundish/short rod-like in G1 and in early to mid-S phase, but prominently elongated/bilobed in late S phase and early G2/M. These morphological traits and segregation pattern of the kinetoplast can be used as a marker for cell cycle stage in a population of asynchronously growing L. donovani promastigotes, in place of cell synchronization procedures or instead of using antibody staining for cell cycle stage marker proteins.