Egf Signaling Directs Neoblast Repopulation by Regulating Asymmetric Cell Division in Planarians

Abstract

A large population of proliferative stem cells (neoblasts) is required for physiological tissue homeostasis and post-injury regeneration in planarians. Recent studies indicate that survival of a few neoblasts after sublethal irradiation results in the clonal expansion of the surviving stem cells and the eventual restoration of tissue homeostasis and regenerative capacity. Yet, the precise mechanisms regulating the population dynamics of neoblasts remain largely unknown. Here, we uncovered a central role for Epidermal Growth Factor (EGF) signaling during in vivo neoblast expansion mediated by Smed-egfr-3 (egfr-3) and its putative ligand Smed-neuregulin-7 (nrg-7). Furthermore, the EGFR-3 protein localizes asymmetrically on the cytoplasmic membrane of neoblasts and the ratio of asymmetric to symmetric cell divisions decreases significantly in egfr-3(RNAi) worms. Our results not only provide the first molecular evidence of asymmetric stem cell divisions in planarians, but also demonstrate that EGF signaling likely functions as an essential regulator of neoblast clonal expansion.

Nervous system development and regeneration in freshwater planarians

“…However, the mature nervous system also expresses genes necessary for proper nervous system regeneration. For instance, ndk , Smed‐egr‐4 , and Smed‐nrg‐7 , are expressed in the nervous system. It is likely that gene expression in existing neural tissues assists in driving the regenerative process by providing additional positional information, and later by providing guidance cues to integrate newly formed or damaged neurons into the existing circuitry.…”

“…The Smed‐nrg‐7 RNAi phenotype recapitulates the effect of Smed‐egfr‐3 knock down. In addition, biochemical analyses indicate that Smed‐nrg‐7 encodes a ligand for Smed‐egfr‐3 , further implicating this ligand‐receptor pair in neoblast cell fate commitment. A recent RNAi screen has also identified putative signaling molecules CRELD, F‐spondin, and LDLRR family genes, all of which encode secreted or transmembrane proteins that could be involved in cell signaling required for brain regeneration …”

“…Nevertheless, some potential factors or downstream effectors have been identified from RNAi screens. The epidermal growth factor receptor (EGFR) signaling pathway is crucial for neoblast regulation and cell fate specification . Smed‐egfr‐3 RNAi inhibits blastema formation and impairs cellular differentiation because of a dysregulation in asymmetric cell division .…”

Utilizing the planarian voltage-gated ion channel transcriptome to resolve a role for a Ca 2+ channel in neuromuscular function and regeneration

“…Ca v 1B mediated Ca 2+ influx could impact muscle function by altering neurotransmission, or by directly impacting muscle Ca 2+ signaling – mRNA localization data suggests expression in both excitable cell types (Figure 4C, (Wurtzel et al, 2015)). Numerous studies evidence that dysregulation of the excitable cell niche by perturbation of CNS or muscle biology can impact regenerative polarity signaling (Yazawa et al, 2009; Oviedo et al, 2010; Zhang et al, 2011; Cowles et al, 2013; Cowles et al, 2014; Durant et al, 2016; Lei et al, 2016), and it seems reasonable to speculate that a broad range of pharmacological agents that miscue regenerative polarity share the capacity to target the excitable cell niche and dysregulate muscle function (Chan et al, 2015). This would provide an unexpected commonality amongst a lengthy historical pharmacopeia of exogenous agents that impact regeneration (Rustia, 1925; Teshirogi, 1955; Kanatani, 1958; Flickinger, 1959; Rodriguez and Flickinger, 1971; Nogi and Levin, 2005; Nogi et al, 2009; Salvetti et al, 2009; Oviedo et al, 2010; Beane et al, 2011) and prioritize further investigation of excitable cell physiology, voltage-operated ion channels and Ca 2+ signals in the control of regenerative outcomes.…”

Feedback regulation of steady-state epithelial turnover and organ size

“…6a-d, k and 7i), whereas enterocyte rho is required for homeostasis (Fig 4) but not repair 46 . 232 Stem cell EGFR signaling is known to affect homeostasis of other tissues [72][73][74] , raising the 233 possibility that spatially specific control of EGFR activation by E-cad and Rho is a general 234 mechanism for cellular equilibrium. By extension, loss of spatial control should lead to patholog-235 ical loss of homeostasis.…”