Changes in ornithine decarboxylase activity and polyamine content and effects of polyamine inhibitors in the regenerating planarianDugesia (G) tigrina

Putrescine independent wound response phenotype is produced by ODC-like RNAi in planarians

Despite increasing evidence indicates polyamines as a convergence point for signaling pathways, including cell growth and differentiation, a unifying concept to interpret their role is still missing. The activity of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, is tightly regulated by a complex molecular machinery, and the demonstration of the existence of multiple ODC paralogs, lacking decarboxylation activity, suggests additional layers of complexity to the intricate ODC regulatory pathway. Because of their extraordinary regenerative abilities and abundance of stem cells, planarians have potential to contribute to our understanding of polyamine function in an in vivo context. We undertook a study on ODC function in planarians and we found six planarian ODCs (ODC1-6). Five out of six ODC homologs carry substitutions of key aminoacids for enzymatic activity, which makes them theoretically unable to decarboxylate ornithine. Silencing of ODC5 and 6 produced a complex phenotype, by prompting animals to an aberrant response, following chronic injury without tissue removal. Phenotype is neither rescued by putrescine, nor mimicked by difluoromethylornithine treatment. Moreover, the co-silencing of other genes of the ODC regulatory pathway did not modulate phenotype outcome or severity, thus suggesting that the function/s of these ODC-like proteins might be unrelated to decarboxylase activity and putrescine production.

Egr-5 is a post-mitotic regulator of planarian epidermal differentiation

Neoblasts are an abundant, heterogeneous population of adult stem cells (ASCs) that facilitate the maintenance of planarian tissues and organs, providing a powerful system to study ASC self-renewal and differentiation dynamics. It is unknown how the collective output of neoblasts transit through differentiation pathways to produce specific cell types. The planarian epidermis is a simple tissue that undergoes rapid turnover. We found that as epidermal progeny differentiate, they progress through multiple spatiotemporal transition states with distinct gene expression profiles. We also identified a conserved early growth response family transcription factor, egr-5, that is essential for epidermal differentiation. Disruption of epidermal integrity by egr-5 RNAi triggers a global stress response that induces the proliferation of neoblasts and the concomitant expansion of not only epidermal, but also multiple progenitor cell populations. Our results further establish the planarian epidermis as a novel paradigm to uncover the molecular mechanisms regulating ASC specification in vivo.

Specific requirement of putrescine for the mitogenic action of juvenile hormone on adult insect neuroblasts

Persistent neurogenesis in an adult insect brain was recently shown to be stimulated by juvenile hormone (JH). This morphogenetic hormone was also shown to act on polyamine biosynthesis. To analyze the possible involvement of polyamines in the neurogenic action of JH, two series of experiments were carried out with adult female crickets, Acheta domesticus: (i) inhibition of the first key enzyme in polyamine biosynthesis, ornithine decarboxylase, with alpha-difluoromethylornithine (alpha-DFMO), and examination of the effects of this treatment on the neuroblast proliferation response to JH; and (ii) examination of the effects of putrescine supplementation on the mitotic index of JH-deprived and alpha-DFMO-treated females. In control females, alpha-DFMO treatment, as well as JH deprivation, greatly reduced neuroblast proliferation. Putrescine supplementation in alpha-DFMO-treated insects overcame the effects of alpha-DFMO, and allowed for detection of putrescine in the neural tissue and stimulation of brain neurogenesis. In JH-deprived females, alpha-DFMO treatment completely prevented the stimulatory action of JH on neuroblast proliferation and on brain putrescine levels. By contrast, putrescine feeding of JH-deprived animals was able to mimic the stimulatory effect of JH: brain putrescine levels increased and neuroblast proliferation was restored. To our knowledge, this report demonstrates for the first time that in vivo administration of putrescine can mimic the effects of a morphogenetic hormone on adult neuroblast proliferation, and shows the importance of polyamines, especially putrescine, in the transduction of JH message in neural tissue.