Not lost in translation

Adult Endogenous Dopaminergic Neuroregeneration Against Parkinson's Disease: Ideal Animal Models?

Parkinson's disease (PD) is the second most common neurodegenerative disease. The etiology of PD remains an enigma with no available disease modifying treatment or cure. Pharmacological compensation is the only quality of life improving treatments available. Endogenous dopaminergic neuroregeneration has recently been considered a plausible therapeutic strategy for PD. However, researchers have to first decipher the complexity of adult endogenous neuroregeneration. This raises the need of animal models to understand the underlying molecular basis. Mammalian models with highly conserved genetic homology might aid researchers to identify specific molecular mechanisms. However, the scarcity of adult neuroregeneration potential in mammals obfuscates such investigations. Nowadays, non-mammalian models are gaining popularity due to their explicit ability to neuroregenerate naturally without the need of external enhancements, yet these non-mammals have a much diverse gene homology that critical molecular signals might not be conserved across species. The present review highlights the advantages and disadvantages of both mammalian and non-mammalian animal models that can be essentially used to study the potential of endogenous DpN regeneration against PD.

Live cell-lineage tracing and machine learning reveal patterns of organ regeneration

Despite the intrinsically stochastic nature of damage, sensory organs recapitulate normal architecture during repair to maintain function. Here we present a quantitative approach that combines live cell-lineage tracing and multifactorial classification by machine learning to reveal how cell identity and localization are coordinated during organ regeneration. We use the superficial neuromasts in larval zebrafish, which contain three cell classes organized in radial symmetry and a single planar-polarity axis. Visualization of cell-fate transitions at high temporal resolution shows that neuromasts regenerate isotropically to recover geometric order, proportions and polarity with exceptional accuracy. We identify mediolateral position within the growing tissue as the best predictor of cell-fate acquisition. We propose a self-regulatory mechanism that guides the regenerative process to identical outcome with minimal extrinsic information. The integrated approach that we have developed is simple and broadly applicable, and should help define predictive signatures of cellular behavior during the construction of complex tissues.

Could we also be regenerative superheroes, like salamanders?

Development of methods to reawaken the semi-dormant regenerative potential that lies within adult human tissues would hold promise for the restoration of diseased or damaged organs and tissues. While most of the regeneration potential is suppressed in many vertebrates, including humans, during adult life, urodele amphibians (salamanders) retain their regenerative ability throughout adulthood. Studies in newts and axolotls, two salamander models, have provided significant knowledge about adult limb regeneration. In this review, we present a comparative analysis of salamander and mammalian regeneration and discuss how evolutionarily altered properties of the regenerative environment can be exploited to restore full regenerative potential in the human body.

Cell death: a program to regenerate

Recent studies in Drosophila, Hydra, planarians, zebrafish, mice, indicate that cell death can open paths to regeneration in adult animals. Indeed injury can induce cell death, itself triggering regeneration following an immediate instructive mechanism, whereby the dying cells release signals that induce cellular responses over short and/or long-range distances. Cell death can also provoke a sustained derepressing response through the elimination of cells that suppress regeneration in homeostatic conditions. Whether common properties support what we name "regenerative cell death," is currently unclear. As key parameters, we review here the injury proapoptotic signals, the signals released by the dying cells, the cellular responses, and their respective timing. ROS appears as a common signal triggering cell death through MAPK and/or JNK pathway activation. But the modes of ROS production vary, from a brief pulse upon wounding, to repeated waves as observed in the zebrafish fin where ROS supports two peaks of cell death. Indeed regenerative cell death can be restricted to the injury phase, as in Hydra, Drosophila, or biphasic, immediate, and delayed, as in planarians and zebrafish. The dying cells release in a caspase-dependent manner a variety of signaling molecules, cytokines, growth factors, but also prostaglandins or ATP as recorded in Drosophila, Hydra, mice, and zebrafish, respectively. Interestingly, the ROS-producing cells often resist to cell death, implying a complex paracrine mode of signaling to launch regeneration, involving ROS-producing cells, ROS-sensing cells that release signaling molecules upon caspase activation, and effector cells that respond to these signals by proliferating, migrating, and/or differentiating.

[Between homeostasis and development, which strategies to regenerate?]

The Hydra model system is well suited to decipher the mechanisms underlying adult regeneration, specifically those that were robust enough to be maintained across evolution. After mid-gastric bissection head regeneration in Hydra relies on apoptosis-induced compensatory proliferation via the release of Wnt3 by the apoptotic interstitial cells and activation of the β-catenin pathway in the surrounding cycling interstitial cells. As apoptosis-induced compensatory proliferation is also at work in Drosophila regenerating imaginal discs, Xenopus tadpole regenerating their tail and mice regenerating their skin or their liver, this mechanism might represent an evolutionarily-conserved way to launch a regenerative response. However after decapitation, the analysis of the activation of the canonical Wnt pathway in decapitated Hydra showed that apoptosis-induced compensatory proliferation does not take place in this context. Given that the proportion of interstitial stem cells is significantly higher in the middle part than in the upper part of the body column, this suggested that the route taken to regenerate a structure as complex as the head dramatically varies according to the homeostatic status of the tissue at the time of injury. From these observations, we propose a tri-modular model for animal regeneration where the first module or "wound healing module" is followed by a transient module named "inducing module of regeneration" that allows the recruitment of the third module named "re-development module", necessary for repatterning the missing structure. We claim that among these three modules, the inducing module of regeneration is the most drastically constrained by the homeostatic conditions of any given tissue or organ at the time of injury and therefore the most variable.

Cell plasticity in homeostasis and regeneration

Over the past decades, genetic analyses performed in vertebrate and invertebrate organisms deciphered numerous cellular and molecular mechanisms deployed during sexual development and identified genetic circuitries largely shared among bilaterians. In contrast, the functional analysis of the mechanisms that support regenerative processes in species randomly scattered among the animal kingdom, were limited by the lack of genetic tools. Consequently, unifying principles explaining how stress and injury can lead to the reactivation of a complete developmental program with restoration of original shape and function remained beyond reach of understanding. Recent data on cell plasticity suggest that beside the classical developmental approach, the analysis of homeostasis and asexual reproduction in adult organisms provides novel entry points to dissect the regenerative potential of a given species, a given organ or a given tissue. As a clue, both tissue homeostasis and regeneration dynamics rely on the availability of stem cells and/or on the plasticity of differentiated cells to replenish the missing structure. The freshwater Hydra polyp provides us with a unique model system to study the intricate relationships between the mechanisms that regulate the maintenance of homeostasis, even in extreme conditions (starvation and overfeeding) and the reactivation of developmental programs after bisection or during budding. Interestingly head regeneration in Hydra can follow several routes according to the level of amputation, suggesting that indeed the homeostatic background dramatically influences the route taken to bridge injury and regeneration.

The Hydra model: disclosing an apoptosis-driven generator of Wnt-based regeneration

The Hydra model system is well suited for the eludication of the mechanisms underlying regeneration in the adult, and an understanding of the core mechanisms is likely to cast light on pathways conserved in other species. Recent detailed analyses of the activation of the Wnt-beta-catenin pathway in bisected Hydra shows that the route taken to regenerate a structure as complex as the head varies dramatically according to the level of the amputation. When decapitation induces direct re-development due to Wnt3 signaling from epithelial cells, head regeneration after mid-gastric section relies first on Wnt3 signaling from interstitial cells, that undergo apoptosis-induced compensatory proliferation, and subsequently on activation of Wnt3 signaling in the epithelial cells. The relative distribution between stem cells and head progenitor cells is strikingly different in these two contexts, indicating that the pre-amputation homeostatic conditions define and constrain the route that bridges wound-healing to the re-development program of the missing structure.