Fig 1 - uploaded by Catherine D Mccusker
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Axolotls continue to grow after sexual maturity. Live images of a hatchling axolotl (∼3 weeks old), young juvenile (∼3 months old), late juvenile (∼5 months old), sexually mature adult (∼1 year old), and a 3-year-old adult show the dramatic increase in size over time.
Source publication
Loss of regenerative capacity is a normal part of aging. However, some organisms, such as the Mexican axolotl, retain striking regenerative capacity throughout their lives. Moreover, the development of age-related diseases is rare in this organism. In this review, we will explore how axolotls are used as a model system to study regenerative process...
Contexts in source publication
Context 1
... maximum life expectancy of an axolotl is estimated to be up to 25 years in captivity. Despite the lack of age-related research in this species, there are observable changes in the body structure of the animal with time. One of these changes is size; the axolotl is an indefinitely growing animal, continuing to increase in size throughout its life (Fig. 1). Another is in the composition of its skeleton; during the larval stages of life, the axolotl skeleton is highly cartilaginous, and this cartilage is replaced by bone as the animal ages. Changes in tissue composition are also evident, such as the limb dermal layer thickening as the animal ages (Fig. 2). Behavior also changes with age. ...
Context 2
... a number of factors. (1) The rate of regeneration slows as the axolotl increases in age, from weeks in larval animals to months in sexually mature adults. This may be a consequence of the animal's size, with smaller animals having less extremity girth and mass relative to adult animals leading to faster wound closure and tissue restoration rates (Fig. 1). (2) With age, the skin of the axolotl thickens and loses flexibility, possibly making it more difficult to form a wound epithelium, as well as increase the likelihood of generating insufficient positional interactions for the generation of a complete limb structure at the site of injury [19]. Additional factors, such as circulating ...
Citations
... Neural regeneration capacity is widely exemplified across animals. The extent of these regenerative capacities ranges from cellular regeneration to whole body reformation, with some of the most extreme examples of nervous system regeneration found in flatworms (e.g., Schmidtea mediterranea) [1,2], cnidarians (e.g., Hydra vulgaris) [3,4,5], the replacement of complete innervated limbs in salamanders (e.g., Ambystoma mexicanum) [6,7] or spinal cord injury recovery in zebrafish (e.g., Danio rerio) [8,9]. By contrast, mammals exhibit limited regenerative abilities, along with a complex immune response that slows neural regrowth [10]. ...
Understanding how neurons regenerate neural circuits following an injury is a fundamental question in neuroscience. Hydra is a powerful model for studying this process because it has significant and reproducible regenerative abilities, a simple and transparent body that allows imaging over time, and established methods for creating animals with cell-type-specific expression of transgenes. In addition, cnidarians such as Hydra split from bilaterians (the group that encompasses most model organisms used in neuroscience) over 500 million years ago, so similarities with other models likely indicates deeply conserved biological processes. Hydra is a long-standing regeneration model and is an emerging model for neuroscience; however, relatively little is known regarding the restoration of neural activity and behavior following significant injury. In this study, we ask if regenerating neurons reach a terminal cell fate and then reform functional neural circuits, or if neural circuits regenerate first and then guide the constituent cells toward their terminal fate. To address this question, we developed a dual-expression transgenic Hydra line that expresses a cell-type-specific red fluorescent protein (tdTomato) in ec5 peduncle neurons, and a calcium indicator (GCaMP7s) in all neurons. This transgenic line allowed us to monitor neural activity while we simultaneously track the reappearance of terminally differentiated ec5 neurons as determined by the expression of tdTomato. Using SCAPE (Swept Confocally Aligned Planar Excitation) microscopy we tracked both calcium activity and expression of tdTomato in 3D with single-cell resolution during regeneration of Hydra aboral end. We observed tdTomato expression in ec5 neurons approximately four hours before the neural activity begins to display synchronized patterns associated with a regenerated neural circuit. These data suggest that regenerating neurons undergo terminal differentiation prior to re-establishing their functional role in the nervous system. The combination of dynamic imaging of neural activity and gene expression during regeneration make Hydra a powerful model system for understanding the key molecular and functional processes involved in neuro-regeneration following injury.
... However, there was no significant difference in the expression of COL1 and 4 in the regenerating and non-regenerating blastemas of P. maculatus. A recent study identified the stem cell factor SALL4 as a critical regulator of scar-free wound healing in axolotl [57] as it regulates the expression of type I and type XII collagens [41]. In the present study, SALL4 expression was the highest 3dpa tadpole blastema and probably prevented the deposition of pro-healing collagen (in this case COL 10). ...
Background
Regeneration studies help to understand the strategies that replace a lost or damaged organ and provide insights into approaches followed in regenerative medicine and engineering. Amphibians regenerate their limbs effortlessly and are indispensable models to study limb regeneration. Xenopus and axolotl are the key models for studying limb regeneration but recent studies on non-model amphibians have revealed species specific differences in regeneration mechanisms.
Results
The present study describes the de novo transcriptome of intact limbs and three-day post-amputation blastemas of tadpoles and froglets of the Asian tree frog Polypedates maculatus, a non-model amphibian species commonly found in India. Differential gene expression analysis between early tadpole and froglet limb blastemas discovered species-specific novel regulators of limb regeneration. The present study reports upregulation of proteoglycans, such as epiphycan, chondroadherin, hyaluronan and proteoglycan link protein 1, collagens 2,5,6, 9 and 11, several tumour suppressors and methyltransferases in the P. maculatus tadpole blastemas. Differential gene expression analysis between tadpole and froglet limbs revealed that in addition to the expression of larval-specific haemoglobin and glycoproteins, an upregulation of cysteine and serine protease inhibitors and downregulation of serine proteases, antioxidants, collagenases and inflammatory genes in the tadpole limbs were essential for creating an environment that would support regeneration. Dermal myeloid cells were GAG+, EPYC+, INMT+, LEF1+ and SALL4+ and seemed to migrate from the unamputated regions of the tadpole limb to the blastema. On the other hand, the myeloid cells of the froglet limb blastemas were few and probably contributed to sustained inflammation resulting in healing.
Conclusions
Studies on non-model amphibians give insights into alternate tactics for limb regeneration which can help devise a plethora of methods in regenerative medicine and engineering.
... The "Internet's favorite" cute axolotls are actually amazing creatures in terms of regeneration ability. An axolotl is able to regenerate its damaged limbs, heart, spinal cord, and even brain and thus rarely has age-related diseases [1]. ...
An axolotl can regenerate part of its brain that was removed.
... Recently, the HIF-1α signaling pathway has been identified as an essential factor in wound healing [140], and it is suggested that decreased regeneration could be related to changes in fibroblast activity [141]. HIF transcriptional activity has been reported to enable fibroblast to myofibroblast differentiation and the production of profibrotic mediators, where HIF signaling acts as an amplifier of IPF [56,113,142]. ...
Hypoxia and hypoxia-inducible factors (HIFs) are essential in regulating several cellular processes, such as survival, differentiation, and the cell cycle; this adaptation is orchestrated in a complex way. In this review, we focused on the impact of hypoxia in the physiopathology of idiopathic pulmonary fibrosis (IPF) related to lung development, regeneration, and repair. There is robust evidence that the responses of HIF-1α and -2α differ; HIF-1α participates mainly in the acute phase of the response to hypoxia, and HIF-2α in the chronic phase. The analysis of their structure and of different studies showed a high specificity according to the tissue and the process involved. We propose that hypoxia-inducible transcription factor 2a (HIF-2α) is part of the persistent aberrant regeneration associated with developing IPF.
... This suggests that more extensive epigenetic remodeling may be required to fully revert Acomys cells to a pluripotent identity. Acomys could possibly be resistant to tumorigenesis, similar to Heterocephalus and the regenerative axolotl salamander 44 , warranting further study into cancer in Acomys. ...
Background: The African spiny mouse ( Acomys ) is an emerging mammalian model for scar-free regeneration, and further study of Acomys could advance the field of regenerative medicine. Isolation of pluripotent stem cells from Acomys would allow for development of transgenic or chimeric animals and in vitro study of regeneration; however, the reproductive biology of Acomys is not well characterized, complicating efforts to derive embryonic stem cells. Thus, we sought to generate Acomys induced pluripotent stem cells (iPSCs) by reprogramming somatic cells back to pluripotency.
Methods: To generate Acomys iPSCs, we attempted to adapt established protocols developed in Mus . We utilized a PiggyBac transposon system to genetically modify Acomys fibroblasts to overexpress the Yamanaka reprogramming factors as well as mOrange fluorescent protein under the control of a doxycycline-inducible TetON operon system.
Results: Reprogramming factor overexpression caused Acomys fibroblasts to undergo apoptosis or senescence. When SV40 Large T antigen (SV40 LT) was added to the reprogramming cocktail, Acomys cells were able to dedifferentiate into pre-iPSCs. Although use of 2iL culture conditions induced formation of colonies resembling Mus PSCs, these Acomys iPS-like cells lacked pluripotency marker expression and failed to form embryoid bodies. An EOS-GiP system was unsuccessful in selecting for bona fide Acomys iPSCs; however, inclusion of Nanog in the reprogramming cocktail along with 5-azacytidine in the culture medium allowed for generation of Acomys iPSC-like cells with increased expression of several naïve pluripotency markers.
Conclusions: There are significant roadblocks to reprogramming Acomys cells, necessitating future studies to determine Acomys -specific reprogramming factor and/or culture condition requirements. The requirement for SV40 LT during Acomys dedifferentiation may suggest that tumor suppressor pathways play an important role in Acomys regeneration and that Acomys may possess unreported cancer resistance.
... Such a switch in transcription regulation may triggers the acceleration of aging and a decline in regeneration ability. One aim to study regeneration in neotenic axolotls is to discover the connection between regeneration, aging and cancer 5,76 . Neoteny in naked mole-rats also results in longevity and cancer resistance rather than regeneration ability 4,77 . ...
The Mexican axolotl (Ambystoma mexicanum) is a well-established tetrapod model for regeneration and developmental studies. Remarkably, neotenic axolotls may undergo metamorphosis, a process that triggers many dramatic changes in diverse organs, accompanied by gradually decline of their regeneration capacity and lifespan. However, the molecular regulation and cellular changes in neotenic and metamorphosed axolotls are still poorly investigated. Here, we develop a single-cell sequencing method based on combinatorial hybridization to generate a tissue-based transcriptomic landscape of the neotenic and metamorphosed axolotls. We perform gene expression profiling of over 1 million single cells across 19 tissues to construct the first adult axolotl cell landscape. Comparison of single-cell transcriptomes between the tissues of neotenic and metamorphosed axolotls reveal the heterogeneity of non-immune parenchymal cells in different tissues and established their regulatory network. Furthermore, we describe dynamic gene expression patterns during limb development in neotenic axolotls. This system-level single-cell analysis of molecular characteristics in neotenic and metamorphosed axolotls, serves as a resource to explore the molecular identity of the axolotl and facilitates better understanding of metamorphosis.
... Morphological regeneration does not only occur in plants but also in animals. For instance, salamanders are capable of regenerating an amputated leg [5]. The simple organisms Hydra and Planaria are capable of complete morphological repair, regardless of which body part is removed [6,7]. ...
Biological systems are very robust to morphological damage, but artificial systems (robots) are currently not. In this paper we present a system based on neural cellular automata, in which locomoting robots are evolved and then given the ability to regenerate their morphology from damage through gradient-based training. Our approach thus combines the benefits of evolution to discover a wide range of different robot morphologies, with the efficiency of supervised training for robustness through differentiable update rules. The resulting neural cellular automata are able to grow virtual robots capable of regaining more than 80\% of their functionality, even after severe types of morphological damage.
... Morphological regeneration does not only occur in plants but also in animals. For instance, salamanders are capable of regenerating an amputated leg [5]. The simple organisms Hydra and Planaria are capable of complete morphological repair, regardless of which body part is removed [6,7]. ...
Biological systems are very robust to morphological damage, but artificial systems (robots) are currently not. In this paper we present a system based on neural cellular automata, in which locomoting robots are evolved and then given the ability to regenerate their morphology from damage through gradient-based training. Our approach thus combines the benefits of evolution to discover a wide range of different robot morphologies, with the efficiency of supervised training for robustness through differentiable update rules. The resulting neural cellular automata are able to grow virtual robots capable of regaining more than 80% of their functionality, even after severe types of morphological damage.
... Several species, including zebrafish and axolotls, can overcome scarring via epimorphic regeneration, which is a process similar to embryonic tissue development in which less differentiated blastemal cells arise and retain positional memory to form new tissues [46,47]. Mammalian species, on the other hand, do not regenerate lost/damaged cutaneous tissue and instead replace it with a dense fibrotic scar [48,49]. ...
The ability of an animal to regenerate lost tissue and body parts has obviously life-saving implications. Understanding how this ability became restricted or active in specific animal lineages will help us understand our own regeneration. According to phylogenic analysis, the glial cell line-derived neurotrophic factor (GDNF) signaling pathway, but not other family members, is conserved in axolotls, a salamander with remarkable regenerative capacity. Furthermore, comparing the pro-regenerative Spiny mouse to its less regenerative descendant, the House mouse, revealed that the GDNF signaling pathway, but not other family members, was induced in regenerating Spiny mice. According to GDNF receptor expression analysis, GDNF may promote hair follicle neogenesis – an important feature of skin regeneration – by determining the fate of dermal fibroblasts as part of new hair follicles. These findings support the idea that GDNF treatment will promote skin regeneration in humans by demonstrating the GDNF signaling pathway's ancestral and cellular nature.
... Even for highly regenerative animals like the Mexican axolotl (McCusker et al., 2015), age is associated with a decline in regenerative capacity (Seifert and Voss, 2013;Vieira et al., 2020;McCusker and Gardiner, 2011). Similarly, in humans, while regenerative capacity of the digit tips persists into adulthood, fidelity of regeneration appears to decline with age . ...
De novo limb regeneration after amputation is restricted in mammals to the distal digit tip. Central to this regenerative process is the blastema, a heterogeneous population of lineage-restricted, dedifferentiated cells that ultimately orchestrates regeneration of the amputated bone and surrounding soft tissue. To investigate skeletal regeneration, we made use of spatial transcriptomics to characterize the transcriptional profile specifically within the blastema. Using this technique, we generated a gene signature with high specificity for the blastema in both our spatial data, as well as other previously published single-cell RNA-sequencing transcriptomic studies. To elucidate potential mechanisms distinguishing regenerative from non-regenerative healing, we applied spatial transcriptomics to an aging model. Consistent with other forms of repair, our digit amputation mouse model showed a significant impairment in regeneration in aged mice. Contrasting young and aged mice, spatial analysis revealed a metabolic shift in aged blastema associated with an increased bioenergetic requirement. This enhanced metabolic turnover was associated with increased hypoxia and angiogenic signaling, leading to excessive vascularization and altered regenerated bone architecture in aged mice. Administration of the metabolite oxaloacetate decreased the oxygen consumption rate of the aged blastema and increased WNT signaling, leading to enhanced in vivo bone regeneration. Thus, targeting cell metabolism may be a promising strategy to mitigate aging-induced declines in tissue regeneration.
