Lifespan extension in a semelparous chordate Oikopleura dioica via developmental growth arrest : Roles of Target of Rapamycin (TOR) signaling and D type cyclins
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All living organisms experience ageing during the course of their lifespan. The science of ageing has emerged into an enthralling area of research. Our knowledge of the mechanisms of ageing and reproduction has been mainly derived from iteroparous model organisms such as C. elegans and D. melanogaster. The mechanisms governing lifespan are linked to signaling from the reproductive tissues. In this study, we examined to what extent these mechanisms are evolutionarily conserved in a semelparous model organism 0ikopleura dioica. This marine chordate is a dioecious species that belongs to the sister group of vertebrates. Rapid growth occurs by increasing the size of somatic endocycling cells. The pre-meiotic gonad consists of mitotic germline nuclei which proliferate asynchronously in a common cytoplasm (syncytium). We identify a reversible developmental growth arrest (GA) in 0. dioica in response to high-density, nutrient-limited conditions, which extends its lifespan up to three-fold. Iteroparous models, sacrifice germ cells that have already entered meiosis, and maintain a reduced number of active germline stem cells (GSCs) under starvation. In contrast, the post-meiotic germline of 0. dioica does not maintain GSCs and GA only occurs prior to meiotic entry. Nutrient limitation encountered after the meiotic entry led to production of reduced numbers of progeny. During GA, nutrient-dependent Target of Rapamycin (TOR) signaling activity was reduced whereas MAPK stress signaling ERK1/2 and p38 and their common downstream effector MSK1 pathways were activated. Chemical inhibition of TOR signaling alone was sufficient to prevent meiotic entry and germline differentiation. Under GA conditions, both ERK1/2 and p38 pathways were activated and shown to be critical for survival. However, chemical inhibition of TOR signaling only activated p38-MSK1, but not the ERK1/2 pathway. TOR signaling differentially regulated mitotic and endocycling cell cycles during GA. Somatic endocycles immediately ceased upon entry into GA, whereas mitotic germline nuclei and intestinal cells gradually arrested over time. This was mirrored on release from GA, mitotic germline cell cycle resumed first, followed by mitotic intestinal cell cycles, and finally, somatic endocycles. Unlike C. elegans and Drosophila GSCs, 0. dioica germline nuclei have a distinct G1 phase and D-type cyclins play a major role in cell cycle regulation. TOR signaling differentially regulated endocycling and mitotic germ nuclei by altering the expression of D-type cyclins, E2Fs and Cyclin-dependent Kinase Inhibitor a (CKIa). Under GA/TOR inhibition, levels of Cyclin Dd and E2F1 were reduced immediately in arrested endocycling cells, whereas they declined more gradually in proliferating germ nuclei. Simultaneously, increased expression of negative cell cycle regulators, CKIa and E2F7, was observed during somatic endocycling cell arrest. CKIa-mediated cell cycle arrest in proliferating germline nuclei was delayed through cytoplasmic sequestration of CKIa by increased levels of the Cyclin Dbf3 splice variant (lacking phosphodegron and Retinoblastoma-binding motifs) in the syncytial cytoplasm. Overall in this investigation, we interpret that the single event of meiotic entry is a definitive signal that lifespan extension can no longer occur in 0. dioica. Moreover, we demonstrated that TOR signaling integrates environmental cues such as nutrient limitation, to differentially regulate cell cycle variants and promote lifespan extension, in response to adverse environmental conditions.