Patel-King RS, King SM
Mol. Biol. Cell 2016 Apr;27(8):1204-9
WDR92 is a highly conserved WD-repeat protein that has been proposed to be involved in apoptosis and also to be part of a prefoldin-like cochaperone complex. We found that WDR92 has a phylogenetic signature that is generally compatible with it playing a role in the assembly or function of specifically motile cilia. To test this hypothesis, we performed an RNAi-based knockdown of WDR92 gene expression in the planarianSchmidtea mediterraneaand were able to achieve a robust reduction in mRNA expression to levels undetectable under our standard RT-PCR conditions. We found that this treatment resulted in a dramatic reduction in the rate of organismal movement that was caused by a switch in the mode of locomotion from smooth, cilia-driven gliding to muscle-based, peristaltic contractions. Although the knockdown animals still assembled cilia of normal length and in similar numbers to controls, these structures had reduced beat frequency and did not maintain hydrodynamic coupling. By transmission electron microscopy we observed that many cilia had pleiomorphic defects in their architecture, including partial loss of dynein arms, incomplete closure of the B-tubule, and occlusion or replacement of the central pair complex by accumulated electron-dense material. These observations suggest that WDR92 is part of a previously unrecognized cytoplasmic chaperone system that is specifically required to fold key components necessary to build motile ciliary axonemes.
Hagstrom D, Cochet-Escartin O, Collins EM
Regeneration (Oxf) 2016 Apr;3(2):65-77
Freshwater planarians, famous for their regenerative prowess, have long been recognized as a valuable in vivo animal model to study the effects of chemical exposure. In this review, we summarize the current techniques and tools used in the literature to assess toxicity in the planarian system. We focus on the planarian’s particular amenability for neurotoxicology and neuroregeneration studies, owing to the planarian’s unique ability to regenerate a centralized nervous system. Zooming in from the organismal to the molecular level, we show that planarians offer a repertoire of morphological and behavioral readouts while also being amenable to mechanistic studies of compound toxicity. Finally, we discuss the open challenges and opportunities for planarian brain regeneration to become an important model system for modern toxicology.
Ross E, Blair D, Guerrero-Hernández C, Sánchez Alvarado A
G3 (Bethesda) 2016;6(5):1191-200
Exploiting the conservation of various features of mitochondrial genomes has been instrumental in resolving phylogenetic relationships. Despite extensive sequence evidence, it has not previously been possible to conclusively resolve some key aspects of flatworm mitochondrial genomes, including generally conserved traits, such as start codons, noncoding regions, the full complement of tRNAs, and whether ATP8 is, or is not, encoded by this extranuclear genome. In an effort to address these difficulties, we sought to determine the mitochondrial transcriptomes and genomes of sexual and asexual taxa of freshwater triclads, a group previously poorly represented in flatworm mitogenomic studies. We have discovered evidence for an alternative start codon, an extended cox1 gene, a previously undescribed conserved open reading frame, long noncoding RNAs, and a highly conserved gene order across the large evolutionary distances represented within the triclads. Our findings contribute to the expansion and refinement of mitogenomics to address evolutionary issues in this diverse group of animals.
Barberán S, Martín-Durán JM, Cebrià F
Sci Rep 2016;6:28071
The EGFR pathway is an essential signaling system in animals, whose core components are the epidermal growth factors (EGF ligands) and their trans-membrane tyrosine kinase receptors (EGFRs). Despite extensive knowledge in classical model organisms, little is known of the composition and function of the EGFR pathway in most animal lineages. Here, we have performed an extensive search for the presence of EGFRs and EGF ligands in representative species of most major animal clades, with special focus on the planarian Schmidtea mediterranea. With the exception of placozoans and cnidarians, we found that the EGFR pathway is potentially present in all other analyzed animal groups, and has experienced frequent independent expansions. We further characterized the expression domains of the EGFR/EGF identified in S. mediterranea, revealing a wide variety of patterns and localization in almost all planarian tissues. Finally, functional experiments suggest an interaction between one of the previously described receptors, Smed-egfr-5, and the newly found ligand Smed-egf-6. Our findings provide the most comprehensive overview to date of the EGFR pathway, and indicate that the last common metazoan ancestor had an initial complement of one EGFR and one putative EGF ligand, which was often expanded or lost during animal evolution.
Stubenhaus BM, Dustin JP, Neverett ER, Beaudry MS, Nadeau LE, Burk-McCoy E, He X, Pearson BJ, Pellettieri J
Porphyrias are disorders of heme metabolism frequently characterized by extreme photosensitivity. This symptom results from accumulation of porphyrins, tetrapyrrole intermediates in heme biosynthesis that generate reactive oxygen species when exposed to light, in the skin of affected individuals. Here we report that in addition to producing an ommochrome body pigment, the planarian flatworm Schmidtea mediterranea generates porphyrins in its subepithelial pigment cells under physiological conditions, and that this leads to pigment cell loss when animals are exposed to intense visible light. Remarkably, porphyrin biosynthesis and light-induced depigmentation are enhanced by starvation, recapitulating a common feature of some porphyrias – decreased nutrient intake precipitates an acute manifestation of the disease. Our results establish planarians as an experimentally tractable animal model for research into the pathophysiology of acute porphyrias, and potentially for the identification of novel pharmacological interventions capable of alleviating porphyrin-mediated photosensitivity or decoupling dieting and fasting from disease pathogenesis.
Wang C, Han XS, Li FF, Huang S, Qin YW, Zhao XX, Jing Q
Cell Discov 2016;2:16029
Pigmentation processes occur from invertebrates to mammals. Owing to the complexity of the pigmentary system, in vivo animal models for pigmentation study are limited. Planarians are capable of regenerating any missing part including the dark-brown pigments, providing a promising model for pigmentation study. However, the molecular mechanism of planarian body pigmentation is poorly understood. We found in an RNA interference screen that a forkhead containing transcription factor, Albino, was required for pigmentation without affecting survival or other regeneration processes. In addition, the body color recovered after termination of Albino double stranded RNA feeding owing to the robust stem cell system. Further expression analysis revealed a spatial and temporal correlation between Albino and pigmentation process. Gene expression arrays revealed that the expression of three tetrapyrrole biosynthesis enzymes, ALAD, ALAS and PBGD, was impaired upon Albino RNA interference. RNA interference of PBGD led to a similar albinism phenotype caused by Albino RNA interference. Moreover, PBGD was specifically expressed in pigment cells and can serve as a pigment cell molecular marker. Our results revealed that Albino controls planarian body color pigmentation dominantly via regulating tetrapyrrole biogenesis. These results identified Albino as the key regulator of the tetrapyrrole-based planarian body pigmentation, suggesting a role of Albino during stem cell-pigment cell fate decision and provided new insights into porphyria pathogenesis.
Lander R, Petersen CP
Mechanisms enabling positional identity re-establishment are likely critical for tissue regeneration. Planarians use Wnt/beta-catenin signaling to polarize the termini of their anteroposterior axis, but little is known about how regeneration signaling restores regionalization along body or organ axes. We identify three genes expressed constitutively in overlapping body-wide transcriptional gradients that control trunk-tail positional identity in regeneration. ptk7 encodes a trunk-expressed kinase-dead Wnt co-receptor, wntP-2 encodes a posterior-expressed Wnt ligand, and ndl-3 encodes an anterior-expressed homolog of conserved FGFRL/nou-darake decoy receptors. ptk7 and wntP-2 maintain and allow appropriate regeneration of trunk tissue position independently of canonical Wnt signaling and with suppression of ndl-3 expression in the posterior. These results suggest that restoration of regional identity in regeneration involves the interpretation and re-establishment of axis-wide transcriptional gradients of signaling molecules.
Currie KW, Brown DD, Zhu S, Xu C, Voisin V, Bader GD, Pearson BJ
BACKGROUND: Freshwater planarians are well known for their regenerative abilities. Less well known is how planarians maintain spatial patterning in long-lived adult animals or how they re-pattern tissues during regeneration. HOX genes are good candidates to regulate planarian spatial patterning, yet the full complement or genomic clustering of planarian HOX genes has not yet been described, primarily because only a few have been detectable by in situ hybridization, and none have given morphological phenotypes when knocked down by RNAi.
RESULTS: Because the planarian Schmidtea mediterranea (S. mediterranea) is unsegmented, appendage less, and morphologically simple, it has been proposed that it may have a simplified HOX gene complement. Here, we argue against this hypothesis and show that S. mediterranea has a total of 13 HOX genes, which represent homologs to all major axial categories, and can be detected by whole-mount in situ hybridization using a highly sensitive method. In addition, we show that planarian HOX genes do not cluster in the genome, yet 5/13 have retained aspects of axially restricted expression. Finally, we confirm HOX gene axial expression by RNA deep-sequencing 6 anterior-posterior “zones” of the animal, which we provide as a dataset to the community to discover other axially restricted transcripts.
CONCLUSIONS: Freshwater planarians have an unappreciated HOX gene complexity, with all major axial categories represented. However, we conclude based on adult expression patterns that planarians have a derived body plan and their asexual lifestyle may have allowed for large changes in HOX expression from the last common ancestor between arthropods, flatworms, and vertebrates. Using our in situ method and axial zone RNAseq data, it should be possible to further understand the pathways that pattern the anterior-posterior axis of adult planarians.
Brandl H, Moon H, Vila-Farré M, Liu SY, Henry I, Rink JC
Nucleic Acids Res. 2016 Jan;44(D1):D764-73
Planarian flatworms are in the midst of a renaissance as a model system for regeneration and stem cells. Besides two well-studied model species, hundreds of species exist worldwide that present a fascinating diversity of regenerative abilities, tissue turnover rates, reproductive strategies and other life history traits. PlanMine (http://planmine.mpi-cbg.de/) aims to accomplish two primary missions: First, to provide an easily accessible platform for sharing, comparing and value-added mining of planarian sequence data. Second, to catalyze the comparative analysis of the phenotypic diversity amongst planarian species. Currently, PlanMine houses transcriptomes independently assembled by our lab and community contributors. Detailed assembly/annotation statistics, a custom-developed BLAST viewer and easy export options enable comparisons at the contig and assembly level. Consistent annotation of all transcriptomes by an automated pipeline, the integration of published gene expression information and inter-relational query tools provide opportunities for mining planarian gene sequences and functions. For inter-species comparisons, we include transcriptomes of, so far, six planarian species, along with images, expert-curated information on their biology and pre-calculated cross-species sequence homologies. PlanMine is based on the popular InterMine system in order to make the rich biology of planarians accessible to the general life sciences research community.
Azimzadeh J, Basquin C
The freshwater planarian Schmidtea mediterranea has recently emerged as a valuable model system to study basal bodies (BBs) and cilia. Planarians are free-living flatworms that use cilia beating at the surface of their ventral epidermis for gliding along substrates. The ventral epidermis is composed of multiciliated cells (MCCs) that are similar to the MCCs in the respiratory airways, the brain ventricles, and the oviducts in vertebrates. In the planarian epidermis, each cell assembles approximately eighty cilia that beat in a coordinate fashion across the tissue. The BBs that nucleate these cilia all assemble de novo during terminal differentiation of MCCs. The genome of the planarian S. mediterranea has been sequenced and efficient methods for targeting gene expression by RNA interference are available. Defects induced by perturbing the expression of BB proteins can be detected simply by analyzing the locomotion of planarians. BBs are present in large numbers and in predictable orientation, which greatly facilitates analyses by immunofluorescence and electron microscopy. The great ease in targeting gene expression and analyzing associated defects allowed to identify a set of proteins required for BB assembly and function in planarian MCCs. Future technological developments, including methods for transgenic expression in planarians and in related species, will achieve turning free-living flatworms into powerful model systems to study MCCs and the associated human pathologies.