Regeneration of lost tissues depends on the precise interpretation of molecular signals that control and coordinate the onset of proliferation, cellular differentiation and cell death. However, the nature of those molecular signals and the mechanisms that integrate the cellular responses remain largely unknown. The planarian flatworm is a unique model in which regeneration and tissue renewal can be comprehensively studied in vivo. The presence of a population of adult pluripotent stem cells combined with the ability to decode signaling after wounding enable planarians to regenerate a complete, correctly proportioned animal within a few days after any kind of amputation, and to adapt their size to nutritional changes without compromising functionality. Here, we demonstrate that the stress-activated c-jun–NH₂–kinase (JNK) links wound-induced apoptosis to the stem cell response during planarian regeneration. We show that JNK modulates the expression of wound-related genes, triggers apoptosis and attenuates the onset of mitosis in stem cells specifically after tissue loss. Furthermore, in pre-existing body regions, JNK activity is required to establish a positive balance between cell death and stem cell proliferation to enable tissue renewal, remodeling and the maintenance of proportionality. During homeostatic degrowth, JNK RNAi blocks apoptosis, resulting in impaired organ remodeling and rescaling. Our findings indicate that JNK-dependent apoptotic cell death is crucial to coordinate tissue renewal and remodeling required to regenerate and to maintain a correctly proportioned animal. Hence, JNK might act as a hub, translating wound signals into apoptotic cell death, controlled stem cell proliferation and differentiation, all of which are required to coordinate regeneration and tissue renewal. Planarians, thanks to their extraordinary regenerative capacity, represent a unique model of animal regeneration. After amputation, new animals regenerate from each individual piece of tissue, leading Dalyell to describe them as “immortal under the edge of the knife” in 1814. Planarians also continuously renew their tissues and adapt their size in accordance with nutritional supply. This amazing plasticity relies on the presence of a population of adult pluripotent stem cell, the neoblasts. However, little is known about the mechanisms that trigger cell responses, such as cell death and division, which are required to regenerate and maintain tissues and organs in response to injury or nutritional challenge. Here, we show that JNK acts as a hub in the coordination of these events. Specifically in response to tissue loss, JNK modulates the expression of wound-related genes, induces the elimination of unnecessary cells by apoptotic cell death and controls cell division in neoblasts. Loss of JNK function results in the deregulation of these processes and prevents regeneration. Moreover, we demonstrate that JNK-dependent apoptosis is crucial to generate proportioned organisms during tissue turnover. Our findings reveal a central mechanism in planarians that senses tissue loss and translates this information into cellular responses leading to regeneration and tissue renewal.

The Wingless/Integrated (Wnt) signaling pathway controls multiple events during development and homeostasis. It comprises multiple branches, mainly classified according to their dependence on β-catenin activation. The Wnt/β-catenin branch is essential for the establishment of the embryonic anteroposterior (AP) body axis throughout the phylogenetic tree. It is also required for AP axis establishment during planarian regeneration. Wnt/β-catenin–independent signaling encompasses several different pathways, of which the most extensively studied is the planar cell polarity (PCP) pathway, which is responsible for planar polarization of cell structures within an epithelial sheet. Dishevelled (Dvl) is the hub of Wnt signaling because it regulates and channels the Wnt signal into every branch. Here, we analyze the role of Schmidtea mediterranea Dvl homologs ( Smed-dvl-1 and Smed-dvl-2 ) using gene silencing. We demonstrate that in addition to a role in AP axis specification, planarian Dvls are involved in at least two different β-catenin–independent processes. First, they are essential for neural connectivity through Smed-wnt5 signaling. Second, Smed-dvl-2 , together with the S. mediterranea homologs of Van-Gogh (Vang) and Diversin (Div), is required for apical positioning of the basal bodies of epithelial cells. These data represent evidence not only of the function of the PCP network in lophotrocozoans but of the involvement of the PCP core elements Vang and Div in apical positioning of the cilia.

The wnt signaling pathway is an intercellular communication mechanism essential in cell-fate specification, tissue patterning and regional-identity specification. A βcatenin-dependent signal specifies the AP (Anteroposterior) axis of planarians, both during regeneration of new tissues and during normal homeostasis. Accordingly, four wnts (posterior wnts) are expressed in a nested manner in central and posterior regions of planarians. We have analyzed the specific role of each posterior wnt and the possible cooperation between them in specifying and patterning planarian central and posterior regions. We show that each posterior wnt exerts a distinct role during re-specification and maintenance of the central and posterior planarian regions, and that the integration of the different wnt signals (βcatenin dependent and independent) underlies the patterning of the AP axis from the central region to the tip of the tail. Based on these findings and data from the literature, we propose a model for patterning the planarian AP axis.

The β-catenin-dependent Wnt pathway exerts multiple context-dependent roles in embryonic and adult tissues. In planarians, β-catenin-1 is thought to specify posterior identities through the generation of an anteroposterior gradient. However, the existence of such a gradient has not been directly demonstrated. Here, we use a specific polyclonal antibody to demonstrate that nuclear β-CATENIN-1 exists as an anteroposterior gradient from the pre-pharyngeal region to the tail of the planarian Schmidtea polychroa. High levels in the posterior region steadily decrease towards the pre-pharyngeal region but then increase again in the head region. During regeneration, β-CATENIN-1 is nuclearized in both anterior and posterior blastemas, but the canonical WNT1 ligand only influences posterior nuclearization. Additionally, β-catenin-1 is required for proper anterior morphogenesis, consistent with the high levels of nuclear β-CATENIN-1 observed in this region. We further demonstrate that β-CATENIN-1 is abundant in developing and differentiated organs, and is particularly required for the specification of the germline. Altogether, our findings provide the first direct evidence of an anteroposterior nuclear β-CATENIN-1 gradient in adult planarians and uncover novel, context-dependent roles for β-catenin-1 during anterior regeneration and organogenesis.

Wnts are secreted glycoproteins involved in a broad range of essential cell functions, including proliferation, migration and cell-fate determination. Recent years have seen substantial research effort invested in elucidating the role of the Wnt signaling pathway in planarians, flatworms with incredible regenerative capacities. In this review, we summarize current knowledge on the role of canonical (β-catenin-dependent) and non-canonical (β-catenin-independent) Wnt signaling in planarians, not only during regeneration, but also during normal homeostasis. We also describe some of the preliminary data that has been obtained regarding the role of these pathways during embryogenesis. Models are proposed to integrate the different results which have been obtained to date and highlight those questions that still remain to be answered.

β-Catenin is a bifunctional protein participating in both cell adhesion and canonical Wnt signaling. In cell adhesion, it bridges the transmembrane cadherin and the actin-binding protein α-catenin and is essential for adherens junction formation, whereas in canonical Wnt signaling, it shuttles between the cytosol and nucleus and functions as an essential transcriptional activator. Schmidtea mediterranea β-catenin-1 was identified as a determinant of antero-posterior polarity during body regeneration by mediating Wnt signaling. Here we show that S. mediterranea β-catenin-2 is specifically expressed in epithelial cells in the gut and pharynx, where it has a putative role in mediating cell adhesion. We show evidence that planarian β-catenin-1 and -2 have distinct biochemical properties. β-Catenin-1 can interact with the components of the canonical Wnt signaling pathway but not with α-catenin, whereas β-catenin-2 interacts with cell adhesion molecules, including E-cadherin and α-catenin, but not with Wnt signaling components. Consistent with their specific function, β-catenin-1 is a potent transcriptional activator, whereas β-catenin-2 has no transcriptional activity. Protein sequence alignment also indicates that the planarian β-catenin-1 and -2 retain distinct critical residues and motifs, which are in agreement with the differences in their biochemical properties. At last, phylogenetic analysis reveals a probable Platyhelminthes- specific structural and functional segregation from which the monofunctional β-catenins evolved. Our results thus identify the first two monofunctional β-catenins in metazoans.