Planarian organizers

Mechanisms and translational applications of regeneration in limbs: From renewable animals to humans

BACKGROUND

The regenerative capacity of organisms declines throughout evolution, and mammals lack the ability to regenerate limbs after injury. Past approaches to achieving successful restoration through pharmacological intervention, tissue engineering, and cell therapies have faced significant challenges.

OBJECTIVES

This review aims to provide an overview of the current understanding of the mechanisms behind animal limb regeneration and the successful translation of these mechanisms for human tissue regeneration.

RESULTS

Particular attention was paid to the Mexican axolotl (Ambystoma mexicanum), the only adult tetrapod capable of limb regeneration. We will explore fundamental questions surrounding limb regeneration, such as how amputation initiates regeneration, how the limb knows when to stop and which parts to regenerate, and how these findings can apply to mammalian systems.

CONCLUSIONS

Given the urgent need for regenerative therapies to treat conditions like diabetic foot ulcers and trauma survivors, this review provides valuable insights and ideas for researchers, clinicians, and biomedical engineers seeking to facilitate the regeneration process or elicit full regeneration from partial regeneration events.

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.

Muscular remodeling and anteroposterior patterning during tapeworm segmentation

BACKGROUND

Tapeworms are parasitic flatworms that independently evolved a segmented body plan, historically confounding comparisons with other animals. Anteroposterior (AP) patterning in free-living flatworms and in tapeworm larvae is associated with canonical Wnt signaling and positional control genes (PCGs) are expressed by their musculature in regionalized domains along the AP axis. Here, we extend investigations of PCG expression to the adult of the mouse bile-duct tapeworm Hymenolepis microstoma, focusing on the growth zone of the neck region and the initial establishment of segmental patterning.

RESULTS

We show that the adult musculature includes new, segmental elements that first appear in the neck and that the spatial patterns of Wnt factors are consistent with expression by muscle cells. Wnt factor expression is highly regionalized and becomes AP-polarized in segments, marking them with axes in agreement with the polarity of the main body axis, while the transition between the neck and strobila is specifically demarcated by the expression domain of a Wnt11 paralog.

CONCLUSION

We suggest that segmentation could originate in the muscular system and participate in patterning the AP axis through regional and polarized expression of PCGs, akin to the gene regulatory networks employed by free-living flatworms and other animals.

DjFARP Contributes to the Regeneration and Maintenance of the Brain through Activation of DjRac1 in Dugesia japonica

FERM, RhoGEF, and Pleckstrin domain protein (FARP) mediated RhoGTPase pathways are involved in diverse biological processes, such as neuronal development and tumorigenesis. However, little is known about their role in neural regeneration. We uncovered for the first time that FARP-Rac1 signaling plays an important role in neural regeneration in Dugesia japonica, a planarian that possesses unparalleled regenerative capacities. The planarian FARP homolog DjFARP was primarily expressed in both intact and regenerating brain and pharynx tissue. Functional studies suggested that downregulation of DjFARP with dsRNA in Dugesia japonica led to smaller brain sizes, defects in brain lateral branches, and loss of cholinergic, GABAergic, and dopaminergic neurons in both intact and regenerating animals. Moreover, the Rho GTPase DjRac1 was shown to play a similar role in neural regeneration and maintenance. Rac1 activation assay showed that DjFARP acts as a guanine nucleotide exchange factor (GEF) for DjRac1. Together, these findings indicate that the brain defects seen in DjFARP knockdown animals may be attributable to DjRac1 inactivation. In conclusion, our study demonstrated that DjFARP-DjRac1 signaling was required for the maintenance and proper regeneration of the brain in Dugesia japonica.

Transforming growth factor-β signalling regulates protoscolex formation in the Echinococcus multilocularis metacestode

The lethal zoonosis alveolar echinococcosis (AE) is caused by tumor-like, infiltrative growth of the metacestode larval stage of the tapeworm Echinococcus multilocularis. We previously showed that the metacestode is composed of posteriorized tissue and that the production of the subsequent larval stage, the protoscolex, depends on re-establishment of anterior identities within the metacestode germinative layer. It is, however, unclear so far how protoscolex differentiation in Echinococcus is regulated. We herein characterized the full complement of E. multilocularis TGFβ/BMP receptors, which is composed of one type II and three type I receptor serine/threonine kinases. Functional analyzes showed that all Echinococcus TGFβ/BMP receptors are enzymatically active and respond to host derived TGFβ/BMP ligands for activating downstream Smad transcription factors. In situ hybridization experiments demonstrated that the Echinococcus TGFβ/BMP receptors are mainly expressed by nerve and muscle cells within the germinative layer and in developing brood capsules. Interestingly, the production of brood capsules, which later give rise to protoscoleces, was strongly suppressed in the presence of inhibitors directed against TGFβ/BMP receptors, whereas protoscolex differentiation was accelerated in response to host BMP2 and TGFβ. Apart from being responsive to host TGFβ/BMP ligands, protoscolex production also correlated with the expression of a parasite-derived TGFβ-like ligand, EmACT, which is expressed in early brood capsules and which is strongly expressed in anterior domains during protoscolex development. Taken together, these data indicate an important role of TGFβ/BMP signalling in Echinococcus anterior pole formation and protoscolex development. Since TGFβ is accumulating around metacestode lesions at later stages of the infection, the host immune response could thus serve as a signal by which the parasite senses the time point at which protoscoleces must be produced. Overall, our data shed new light on molecular mechanisms of host-parasite interaction during AE and are relevant for the development of novel treatment strategies.

Wnt/β-catenin signalling is required for pole-specific chromatin remodeling during planarian regeneration

For successful regeneration, the identity of the missing tissue must be specified according to the pre-existing tissue. Planarians are ideal for the study of the mechanisms underlying this process; the same field of cells can regrow a head or a tail according to the missing body part. After amputation, the differential activation of the Wnt/β-catenin signal specifies anterior versus posterior identity. Initially, both wnt1 and notum (Wnt inhibitor) are expressed in all wounds, but 48 hours later they are restricted to posterior or anterior facing wounds, respectively, by an unknown mechanism. Here we show that 12 hours after amputation, the chromatin accessibility of cells in the wound region changes according to the polarity of the pre-existing tissue in a Wnt/β-catenin-dependent manner. Genomic analyses suggest that homeobox transcription factors and chromatin-remodeling proteins are direct Wnt/β-catenin targets, which trigger the expression of posterior effectors. Finally, we identify FoxG as a wnt1 up-stream regulator, probably via binding to its first intron enhancer region.

Djptpn11 is indispensable for planarian regeneration by affecting early wound response genes expression and the Wnt pathway

Planarian is an ideal model system of studying regeneration. Stem cell system and positional control genes (PCGs) are two important factors for perfect regeneration of planarians and they combine to promote their regeneration. Even so, how wounds regulate proliferation and neoblast fate is still important areas to address. Ptpn11 (Protein tyrosine phosphatase non-receptor type 11), one of PTP (Protein tyrosine phosphatase) family members, plays an important role in cellular processes including cell survival, proliferation, differentiation and apoptosis. Nevertheless, the role of ptpn11 in the planarian regeneration has not been fully studied. In this study, we identify the Djptpn11 gene to observe its function in planarian regeneration. The results reveal that the regeneration is severely inhibited and cause the disorder homeostasis in planarians. Furthermore, the stem cells proliferation and differentiation decreases while the apoptosis increases following Djptpn11 RNAi. At the same time, Djptpn11 affects the expression levels of early wound response genes (Djegr2, Dj1-jun, Djrunt1, Djwnt1 and Djnotum). Djwnt1 and Djnotum are two key Wnt signaling pathway genes and Djptpn11 affects the expression levels of Djwnt1 and Djnotum in the early and late stages of planarian regeneration. In general, Djptpn11 is indispensable for the homeostasis and regeneration of planarian by affecting the stem cells, early wound response genes and the Wnt pathway.

DDX24 is required for muscle fiber organization and the suppression of wound-induced Wnt activity necessary for pole re-establishment during planarian regeneration

Planarians have a remarkable ability to undergo whole-body regeneration. Successful regeneration outcome is determined by processes like polarity establishment at the wound site, which is followed by pole (organizer) specification. Interestingly, these determinants are almost exclusively expressed by muscles in these animals. However, the molecular toolkit that enables the functional versatility of planarian muscles remains poorly understood. Here we report that SMED_DDX24, a D-E-A-D Box RNA helicase, is necessary for planarian survival and regeneration. We found that DDX24 is enriched in muscles and its knockdown disrupts muscle fiber organization. This leads to defective pole specification, which in turn results in misregulation of many positional control genes specifically during regeneration. ddx24 RNAi also upregulates wound-induced Wnt signalling. Suppressing this ectopic Wnt activity rescues the knockdown phenotype by enabling better anterior pole regeneration. To summarize, our work highlights the role of an RNA helicase in muscle fiber organization, and modulating amputation-induced wnt levels, both of which seem critical for pole re-organization, thereby regulating whole-body regeneration.

Principles of regeneration revealed by the planarian eye

One approach to elucidating the principles of regeneration is to investigate mechanisms that regenerate a target organ. Planarian eyes are discrete, visible structures that are dispensable for viability, making them powerful for studying the logic of regeneration. Fate specification in eye regeneration occurs in stem cells (neoblasts), generating eye progenitors. Eye progenitor production is not responsive to the presence or absence of the eye, with regeneration explained by constant progenitor production in the appropriate positional environment. Eye progenitors display coarse spatial specification. A combination of eye-extrinsic cues and self-organization with differentiated eye cells dictate where migratory eye progenitors target. Finally, guidepost-like cells influence regenerating axons to facilitate the restoration of eye circuitry. These findings from the eye as a case study present a model that explains how regeneration can occur.

Composite morphogenesis during embryo development

Morphogenesis drives the formation of functional living shapes. Gene expression patterns and signaling pathways define the body plans of the animal and control the morphogenetic processes shaping the embryonic tissues. During embryogenesis, a tissue can undergo composite morphogenesis resulting from multiple concomitant shape changes. While previous studies have unraveled the mechanisms that drive simple morphogenetic processes, how a tissue can undergo multiple and simultaneous changes in shape is still not known and not much explored. In this chapter, we focus on the process of concomitant tissue folding and extension that is vital for the animal since it is key for embryo gastrulation and neurulation. Recent pioneering studies focus on this problem highlighting the roles of different spatially coordinated cell mechanisms or of the synergy between different patterns of gene expression to drive composite morphogenesis.

Early eukaryotic origins and metazoan elaboration of MAPR family proteins

The membrane-associated progesterone receptor (MAPR) family consists of heme-binding proteins containing a cytochrome b₅ (cytb₅) domain characterized by the presence of a MAPR-specific interhelical insert region (MIHIR) between helices 3 and 4 of the canonical cytb5-domain fold. Animals possess three MAPR genes (PGRMC-like, Neuferricin and Neudesin). Here we show that all three animal MAPR genes were already present in the common ancestor of the opisthokonts (comprising animals and fungi as well as related single-celled taxa). All three MAPR genes acquired extensions C-terminal to the cytb₅ domain, either before or with the evolution of animals. The archetypical MAPR protein, progesterone receptor membrane component 1 (PGRMC1), contains phosphorylated tyrosines Y139 and Y180. The combination of Y139/Y180 appeared in the common ancestor of cnidarians and bilaterians, along with an early embryological organizer and synapsed neurons, and is strongly conserved in all bilaterian animals. A predicted protein interaction motif in the PGRMC1 MIHIR is potentially regulated by Y139 phosphorylation. A multilayered model of animal MAPR function acquisition includes some pre-metazoan functions (e.g., heme binding and cytochrome P450 interactions) and some acquired animal-specific functions that involve regulation of strongly conserved protein interaction motifs acquired by animals (Metazoa). This study provides a conceptual framework for future studies, against which especially PGRMC1's multiple functions can perhaps be stratified and functionally dissected.

Temporal, spatial, and genetic regulation of external genitalia development

Fertilization requires the physical combination of gametes, and terrestrial mammals necessitated the evolution of genitalia capable of successfully completing the fertilization process in a non-aqueous environment. Thus, the male mammalian external genitalia evolved as an outgrowth from the body, an appendage sufficient to fertilize eggs housed deep inside the female. In this way, sexual dimorphism of mammalian genitalia became highly pronounced. This highly complex evolutionary divergence both from aqueous fertilization, as well as divergence between the sexes of terrestrial mammals, required exquisitely coordinated, novel patterns of gene expression to regulate the spatial and temporal events governing external genitalia development. Recent studies delineating the genetic regulation of external genitalia development, largely focusing on development of the murine genital tubercle, have vastly enlightened the field of reproductive developmental biology. Murine homologs of human genes have been selectively deleted in the mouse, either in the whole body or using tissue-specific and temporally-specific genetic drivers. The defects in outgrowth and urethral tubularization subsequent to the deletion of specific genes in the developing murine external genitalia delineates which genes are required in which compartments and at what times. This review details how these murine genetic models have created a somewhat modest but rapidly growing library of knowledge detailing the spatial-temporal genetic regulation of external genitalia development.