Cell cycle-dependent expression of a hairy and Enhancer of split (hes) homolog during cleavage and segmentation in leech embryos.

We have cloned genes related to hairy and Enhancer of split (hes) from glossiphoniid leeches, Helobdella robusta and Theromyzon rude. In leech, segments arise sequentially in anteroposterior progression from a posterior growth zone that consists of five bilaterally paired embryonic stem cells called teloblasts. Each teloblast gives rise to segmental founder cells (primary blast cells) that contribute iterated sets of definitive progeny in each segment. Thus, in leech, the "segmentation clock," is closely identified with the cell cycle clock of the teloblasts. We have characterized normal expression patterns of mRNA and protein for the H. robusta hes-class gene (Hro-hes). Semiquantitative RT-PCR revealed that Hro-hes mRNA levels peak while the teloblasts are actively producing primary blast cells. RT-PCR, in situ hybridization and immunostaining revealed that Hro-hes is expressed as early as the first zygotic mitosis and throughout early development. Hro-hes is expressed in macromeres, pro-teloblasts, teloblasts and primary blast cells. HRO-HES protein is localized in the nuclei of cells expressing HRO-HES during interphase; nuclear HRO-HES is reduced during mitosis. In contrast, Hro-hes is transcribed during mitosis and its transcripts are associated with mitotic apparatus (MA). Thus, Hro-hes transcription cycles in antiphase to the nuclear localization of HRO-HES protein. These results indicate that Hro-hes expression, and thus possibly its biological activity, is linked to the cell cycle.