Role of MEKK1 in the anterior‐posterior patterning during planarian regeneration

Abstract

Planarians have established a unique body pattern along the anterior-posterior (AP) axis, which consists of at least four distinct body regions arranged in an anterior to posterior sequence: head, prepharyngeal, pharyngeal (containing a pharynx), and tail regions, and possess high regenerative ability. How they reconstruct the regional continuity in a head-to-tail sequence after amputation still remains unknown. We use as a model planarian Dugesia japonica head regeneration from tail fragments, which involves dynamic rearrangement of the body regionality of preexisting tail tissues along the AP axis, and show here that RNA interference of the gene D. japonica mek kinase 1 (Djmekk1) caused a significant anterior shift in the position of pharynx regeneration at the expense of the prepharyngeal region, while keeping the head region relatively constant in size, and accordingly led to development of a relatively longer tail region. Our data suggest that DjMEKK1 regulates anterior extracellular signal-regulated kinase (ERK) and posterior β-catenin signaling pathways in a positive and negative manner, respectively, to establish a proper balance resulting in the regeneration of planarian's scale-invariant trunk-to-tail patterns across individuals. Furthermore, we demonstrated that DjMEKK1 negatively modulates planarian β-catenin activity via its serine/threonine kinase domain, but not its PHD/RING finger domain, by testing secondary axis formation in Xenopus embryos. The data suggest that Djmekk1 plays an instructive role in the coordination between the establishment of the prepharyngeal region and posteriorizing of pharynx formation by balancing the two opposing morphogenetic signals along the AP axis during planarian regeneration.

Identification and characterization of a fibroblast growth factor gene in the planarian Dugesia japonica

“…We therefore injected the liposome‐encapsulated dsRNA into the gut, and this resulted in a significant silencing effect on Djfgf expression (63% reduction; Figure 10) at 24 hr after amputation. Under this condition, these dsRNA‐treated animals showed a minor decrease in the expression level of DjmkpA (25% reduction; Figure 10), which acts as a readout gene of ERK signaling during planarian regeneration (Hosoda et al., 2018; Tasaki et al., 2011; Umesono et al., 2013). However, we could not detect morphologically obvious regeneration defects in Djfgf(RNAi) regenerates (data not shown).…”

A MEKK1 – JNK mitogen activated kinase (MAPK) cascade module is active in Echinococcus multilocularis stem cells

“…To this end, we first mined the available E. multilocularis genome information [27] by BLASTP analyses using human MEKK1 as a query and yielded as best hit a protein encoded by locus EmuJ_000389600 (annotated as a MAPKKK). Next, we used in similar BLASTP analyses a previously described MEKK1 protein of planaria [26] and, again, obtained EmuJ_000389600 as the gene encoding a protein with highest similarities. Informed by the E. multilocularis genome sequence we then designed primers to clone and sequence the fulllength EmuJ_000389600 cDNA from metacestode mRNA preparations, thus verifying the annotated version as depicted in WormBase Parasite.…”

“…In mammals, one of the decisive upstream activators of JNK signalling is the multifunctional MAPKKK MEKK1 the hallmark of which, in addition to the kinase domain, is a RING motif that directs ubiquitination of target proteins [22]. Interestingly, mekk1 orthologs appear to be absent from well-studied invertebrate model systems such as Drosophila and C. elegans but at least one such molecule has been described in the free-living flatworm Dugesia japonica [26]. We now could clearly show that also the parasitic flatworm E. multilocularis expresses a mekk1 ortholog, emmekk1, and that the respective gene is well expressed in the majority of EdU+ (proliferating) stem cells of the parasite.…”

“…One of the best studied pathways leading to JNK activation involves direct binding of the MAPKKK MEKK1 to JNK after stimulation of upstream EGF receptors, using Grb2 (Growth factor receptor bound 2) as an adapter molecule between the EGF receptor and MEKK1 [18][19][20][21][22]. Interestingly, in free-living planarians, which are closely related but non-parasitic relatives to cestodes, both orthologs to MEKK1 and JNK are involved in processes of body axis formation and stem cell (neoblast) maintenance [23][24][25][26]. Given that the E. multilocularis metacestode is a result of modified body axis formation [5] and that neoblast-like stem cells are central to the proliferation of Echinococcus larvae [2], this prompted us to closer characterize this specific branch of MAPK signalling in parasite larvae.…”

Wound repair, remodeling, and regeneration

“…With articles on “recognition of positional information”, Hosoda et al. () and Shibata, Kawasaki, Sakai, and Kawakami () report cell signaling and positional information in planarian regeneration and zebrafish fin regeneration, respectively. In addition, Morioka, Mohanty‐Hejmadi, Yaoita, and Tazawa () review homeotic transformation of tails into limbs in frogs.…”