BMP suppresses WNT to integrate patterning of orthogonal body axes in adult planarians

Pluripotent Stem Cell Plasticity is Sculpted by a Slit-Independent Robo Pathway in a Regenerative Animal

Whole-body regeneration requires adult stem cells with high plasticity to differentiate into missing cell types. Planarians possess a unique configuration of organs embedded in a vast pool of pluripotent stem cells. How stem cells integrate positional information with discrete fates remains unknown. Here, we use the planarian pharynx to define the cell fates that depend on the pioneer transcription factor FoxA. We find that Roundabout receptor RoboA suppresses aberrant pharynx cell fates by altering foxA expression, independent of the canonical ligand Slit. An RNAi screen for extracellular proteins identifies Anosmin-1 as a potential partner of RoboA. Perturbing global patterning demonstrates that roboA / anosmin-1 functions locally in the brain. By contrast, altering pharynx fate with foxA knockdown induces head-specific neurons in the pharynx, indicating a latent plasticity of stem cells. Our data links critical extracellular cues with cell fate decisions of highly plastic stem cells, ensuring the fidelity of organ regeneration.

Stem cells (neoblasts) and positional information jointly dominate regeneration in planarians

Regeneration is the ability to accurately regrow missing body parts. The unparalleled regenerative capacity and incredible tissue plasticity of planarians, both resulting from the presence of abundant adult stem cells referred to as neoblasts, offer a unique opportunity to investigate the cellular and molecular principles underlying regeneration. Neoblasts are capable of self-renewal and differentiation into the desired cell types for correct replacement of lost parts after tissue damage. Positional information in muscle cells governs the polarity and patterning of the body plan during homeostasis and regeneration. For planarians, removal of neoblasts disables the regenerative feats and disruption of positional information results in the regeneration of inappropriate missing body regions, only the combination of neoblasts and positional information enables regeneration. Here, I summarize the current state of the field in neoblast lineage potential, subclasses and specification, and in the roles of positional information for proper tissue turnover and regeneration in planarians.

Injury-induced Neuregulin-ErbB signaling from muscle mobilizes stem cells for whole-body regeneration in Acoels

The activation of progenitor cells near wound sites is a common feature of regeneration across species, but the conserved signaling mechanisms responsible for this step in whole-body regeneration are still incompletely understood. The acoel Hofstenia miamia undergoes whole-body regeneration using Piwi+ pluripotent adult stem cells (neoblasts) that accumulate at amputation sites early in the regeneration process. The EGFR signaling pathway has broad roles in controlling proliferation, migration, differentiation, and cell survival across metazoans. Using a candidate RNAi screening approach, we identify the Hofstenia EGFR erbB4-2 and Neuregulin nrg-1 genes as essential for blastema formation. Structure prediction of NRG-1 and ERBB4-2 proteins supports the likelihood of these factors interacting directly. After amputation injuries, nrg-1 expression is induced in body-wall muscle cells at the wound site by 6 hours and localizes to the tip of the outgrowing blastema over the next several days, while erbB4-2 is broadly expressed, including in muscle and neoblasts. Under nrg-1(RNAi) and erbB4-2(RNAi) conditions that impair blastema formation, animals still undergo the earliest responses to injury to activate expression of the Early Growth Response transcription factor egr, indicating a crucial role for EGFR signaling downstream of initial wound activation. nrg-1(RNAi) and erbB4-2(RNAi) animals possess Piwi+ and H3P+ mitotic neoblasts which hyperproliferate normally after amputation, but these cells fail to accumulate at the wound site. Therefore, muscle provides a source for Neuregulin-ErbB signaling necessary for the mobilization of proliferative progenitors to enable blastema outgrowth for whole-body regeneration in Hofstenia. These results indicate a shared functional requirement for muscle signaling to enable regeneration between planarians and acoels across 550 million years of evolution.

'Three signals - three body axes' as patterning principle in bilaterians

In vertebrates, the three orthogonal body axes, anteroposterior (AP), dorsoventral (DV) and left-right (LR) are determined at gastrula and neurula stages by the Spemann-Mangold organizer and its equivalents. A common feature of AP and DV axis formation is that an evolutionary conserved interplay between growth factors (Wnt, BMP) and their extracellular antagonists (e.g. Dkk1, Chordin) creates signaling gradients for axial patterning. Recent work showed that LR patterning in Xenopus follows the same principle, with R-spondin 2 (Rspo2) as an extracellular FGF antagonist, which creates a signaling gradient that determines the LR vector. That a triad of anti-FGF, anti-BMP, and anti-Wnt governs LR, DV, and AP axis formation reveals a unifying principle in animal development. We discuss how cross-talk between these three signals confers integrated AP-DV-LR body axis patterning underlying developmental robustness, size scaling, and harmonious regulation. We propose that Urbilateria featured three orthogonal body axes that were governed by a Cartesian coordinate system of orthogonal Wnt/AP, BMP/DV, and FGF/LR signaling gradients.

Sizzled (Frzb3) physically interacts with noncanonical Wnt ligands to inhibit gastrulation cell movement

The coordinated movement of germ layer progenitor cells reaches its peak at the dorsal side, where the Bmp signaling gradient is low, and minimum at the ventral side, where the Bmp gradient is high. This dynamic cell movement is regulated by the interplay of various signaling pathways. The noncanonical Wnt signaling cascade serves as a pivotal regulator of convergence and extension cell movement, facilitated by the activation of small GTPases such as Rho, Rab, and Rac. However, the underlying cause of limited cell movement at the ventral side remains elusive. To explore the functional role of a key regulator in constraining gastrulation cell movement at the ventral side, we investigated the Bmp4-direct target gene, sizzled (szl), to assess its potential role in inhibiting noncanonical Wnt signaling. In our current study, we demonstrated that ectopic expression of szl led to gastrulation defects in a dose-dependent manner without altering cell fate specification. Overexpression of szl resulted in decreased elongation of Activin-treated animal cap and Keller explants. Furthermore, our immunoprecipitation assay unveiled the physical interaction of Szl with noncanonical Wnt ligand proteins (Wnt5 and Wnt11). Additionally, the activation of small GTPases involved in Wnt signaling mediation (RhoA and Rac1) was diminished upon szl overexpression. In summary, our findings suggest that Bmp4 signaling negatively modulates cell movement from the ventral side of the embryo by inducing szl expression during early Xenopus gastrulation.

Highly conserved and extremely evolvable: BMP signalling in secondary axis patterning of Cnidaria and Bilateria

Bilateria encompass the vast majority of the animal phyla. As the name states, they are bilaterally symmetric, that is with a morphologically clear main body axis connecting their anterior and posterior ends, a second axis running between their dorsal and ventral surfaces, and with a left side being roughly a mirror image of their right side. Bone morphogenetic protein (BMP) signalling has widely conserved functions in the formation and patterning of the second, dorso-ventral (DV) body axis, albeit to different extents in different bilaterian species. Whilst initial findings in the fruit fly Drosophila and the frog Xenopus highlighted similarities amongst these evolutionarily very distant species, more recent analyses featuring other models revealed considerable diversity in the mechanisms underlying dorsoventral patterning. In fact, as phylogenetic sampling becomes broader, we find that this axis patterning system is so evolvable that even its core components can be deployed differently or lost in different model organisms. In this review, we will try to highlight the diversity of ways by which BMP signalling controls bilaterality in different animals, some of which do not belong to Bilateria. Future research combining functional analyses and modelling is bound to give us some understanding as to where the limits to the extent of the evolvability of BMP-dependent axial patterning may lie.

Mechanistic regulation of planarian shape during growth and degrowth

Adult planarians can grow when fed and degrow (shrink) when starved while maintaining their whole-body shape. It is unknown how the morphogens patterning the planarian axes are coordinated during feeding and starvation or how they modulate the necessary differential tissue growth or degrowth. Here, we investigate the dynamics of planarian shape together with a theoretical study of the mechanisms regulating whole-body proportions and shape. We found that the planarian body proportions scale isometrically following similar linear rates during growth and degrowth, but that fed worms are significantly wider than starved worms. By combining a descriptive model of planarian shape and size with a mechanistic model of anterior-posterior and medio-lateral signaling calibrated with a novel parameter optimization methodology, we theoretically demonstrate that the feedback loop between these positional information signals and the shape they control can regulate the planarian whole-body shape during growth. Furthermore, the computational model produced the correct shape and size dynamics during degrowth as a result of a predicted increase in apoptosis rate and pole signal during starvation. These results offer mechanistic insights into the dynamic regulation of whole-body morphologies.