Differential regulation of fibroblast growth factor receptors in the regenerating amphibian spinal cord in vivo

Tail and Spinal Cord Regeneration in Urodelean Amphibians

Urodelean amphibians can regenerate the tail and the spinal cord (SC) and maintain this ability throughout their life. This clearly distinguishes these animals from mammals. The phenomenon of tail and SC regeneration is based on the capability of cells involved in regeneration to dedifferentiate, enter the cell cycle, and change their (or return to the pre-existing) phenotype during de novo organ formation. The second critical aspect of the successful tail and SC regeneration is the mutual molecular regulation by tissues, of which the SC and the apical wound epidermis are the leaders. Molecular regulatory systems include signaling pathways components, inflammatory factors, ECM molecules, ROS, hormones, neurotransmitters, HSPs, transcriptional and epigenetic factors, etc. The control, carried out by regulatory networks on the feedback principle, recruits the mechanisms used in embryogenesis and accompanies all stages of organ regeneration, from the moment of damage to the completion of morphogenesis and patterning of all its structures. The late regeneration stages and the effects of external factors on them have been poorly studied. A new model for addressing this issue is herein proposed. The data summarized in the review contribute to understanding a wide range of fundamentally important issues in the regenerative biology of tissues and organs in vertebrates including humans.

Regulation of stem cell identity by miR-200a during spinal cord regeneration

Axolotls are an important model organism for multiple types of regeneration, including functional spinal cord regeneration. Remarkably, axolotls can repair their spinal cord after a small lesion injury and can also regenerate their entire tail following amputation. Several classical signaling pathways that are used during development are reactivated during regeneration, but how this is regulated remains a mystery. We have previously identified miR-200a as a key factor that promotes successful spinal cord regeneration. Here, using RNA-seq analysis, we discovered that the inhibition of miR-200a results in an upregulation of the classical mesodermal marker brachyury in spinal cord cells after injury. However, these cells still express the neural stem cell marker sox2. In vivo cell tracking allowed us to determine that these cells can give rise to cells of both the neural and mesoderm lineage. Additionally, we found that miR-200a can directly regulate brachyury via a seed sequence in the 3′UTR of the gene. Our data indicate that miR-200a represses mesodermal cell fate after a small lesion injury in the spinal cord when only glial cells and neurons need to be replaced.

Summary: Axolotl spinal cord cells have the potential to form cells of the ectoderm and mesoderm depending on the extent of the injury they are responding to.

Spatiotemporal Profile and Morphological Changes of NG2 Glia in the CA1 Region of the Rat Hippocampus after Transient Forebrain Ischemia

Neuron-glial antigen-2 (NG2) glia undergo proliferation and morphological changes following brain insults. Here, we show that NG2 glia is activated in a characteristic time- and layer-specific manner in the ischemia-vulnerable CA1 region of the rat hippocampus. Resting NG2 glia of the pyramidal cell layer (somatic region) shared morphological features with those of the neighboring dendritic stratum radiatum. During the postischemic period, reactive NG2 glia of the pyramidal cell layer exhibited shortened, scarcely branched processes, while those of the stratum radiatum had multiple branching processes with their arborization being almost indiscernible 7~14 days after reperfusion. Immunoelectron microscopy demonstrated that NG2 immunoreactivity was specifically associated with the plasma membrane and the adjacent extracellular matrix of NG2 glia in the stratum radiatum at 14 days. NG2 glia also exhibited differences in their numbers and proliferation profiles in the two examined hippocampal strata after ischemia. In addition, induced NG2 expression in activated microglia/macrophages exhibited a characteristic strata-dependent pattern in the ischemic CA1 hippocampus. NG2 induction was prominent in macrophage-like phenotypes which were predominantly localized in the pyramidal cell layer, compared with activated stellate microglial cells in the stratum radiatum. Thus, our data demonstrate that activation of NG2 glia and the induction of NG2 expression in activated microglia/macrophages occur in a distinct time- and layer-specific manner in the ischemic CA1 hippocampus. These characteristic profiles of reactive NG2 glia could be secondary to the degeneration processes occurring in the cell bodies or dendritic domains of hippocampal CA1 pyramidal neurons after ischemic insults.

A comprehensive reference transcriptome resource for the Iberian ribbed newt Pleurodeles waltl, an emerging model for developmental and regeneration biology

Urodele newts have unique biological properties, notably including prominent regeneration ability. The Iberian ribbed newt, Pleurodeles waltl, is a promising model amphibian distinguished by ease of breeding and efficient transgenic and genome editing methods. However, limited genetic information is available for P. waltl. We conducted an intensive transcriptome analysis of P. waltl using RNA-sequencing to build and annotate gene models. We generated 1.2 billion Illumina reads from a wide variety of samples across 12 different tissues/organs, unfertilized egg, and embryos at eight different developmental stages. These reads were assembled into 1,395,387 contigs, from which 202,788 non-redundant ORF models were constructed. The set is expected to cover a large fraction of P. waltl protein-coding genes, as confirmed by BUSCO analysis, where 98% of universal single-copy orthologs were identified. Ortholog analyses revealed the gene repertoire evolution of urodele amphibians. Using the gene set as a reference, gene network analysis identified regeneration-, developmental-stage-, and tissue-specific co-expressed gene modules. Our transcriptome resource is expected to enhance future research employing this emerging model animal for regeneration research as well as for investigations in other areas including developmental biology, stem cell biology, and cancer research. These data are available via our portal website, iNewt (http://www.nibb.ac.jp/imori/main/).

Spinal Cord Regeneration in Amphibians: A Historical Perspective

In some vertebrates, a grave injury to the central nervous system (CNS) results in functional restoration, rather than in permanent incapacitation. Understanding how these animals mount a regenerative response by activating resident CNS stem cell populations is of critical importance in regenerative biology. Amphibians are of a particular interest in the field because the regenerative ability is present throughout life in urodele species, but in anuran species it is lost during development. Studying amphibians, who transition from a regenerative to a nonregenerative state, could give insight into the loss of ability to recover from CNS damage in mammals. Here, we highlight the current knowledge of spinal cord regeneration across vertebrates and identify commonalities and differences in spinal cord regeneration between amphibians.

Planar cell polarity-mediated induction of neural stem cell expansion during axolotl spinal cord regeneration

Axolotls are uniquely able to mobilize neural stem cells to regenerate all missing regions of the spinal cord. How a neural stem cell under homeostasis converts after injury to a highly regenerative cell remains unknown. Here, we show that during regeneration, axolotl neural stem cells repress neurogenic genes and reactivate a transcriptional program similar to embryonic neuroepithelial cells. This dedifferentiation includes the acquisition of rapid cell cycles, the switch from neurogenic to proliferative divisions, and the re-expression of planar cell polarity (PCP) pathway components. We show that PCP induction is essential to reorient mitotic spindles along the anterior-posterior axis of elongation, and orthogonal to the cell apical-basal axis. Disruption of this property results in premature neurogenesis and halts regeneration. Our findings reveal a key role for PCP in coordinating the morphogenesis of spinal cord outgrowth with the switch from a homeostatic to a regenerative stem cell that restores missing tissue.

DOI:http://dx.doi.org/10.7554/eLife.10230.001

Stem cells found in adult tissues are vitally important for tissue repair and maintenance. These cells divide in two main ways: equally to create two new stem cells, or unequally to create a stem cell and a cell that can develop into one of the cell types in the tissue. A key challenge for biologists is to understand how these tissue-resident stem cells are activated and organized to regenerate injured or missing tissue.

Throughout the life of the axolotl salamander, neural stem cells in the spinal cord occasionally divide to add new nerve cells to the healthy spinal cord. However, the axolotl can also regenerate part of its spinal cord, for example if its tail is lost. Under these conditions, the neural stem cells can convert into a highly regenerative stem cell that can produce all the different cell types needed to regrow the spinal cord.

As a stem cell becomes a new cell type, it activates different sets of genes. Therefore, Rodrigo Albors, Tazaki et al. measured gene activity in the neural stem cells involved in axolotl spinal cord regeneration to uncover how these cells develop into a more regenerative form. This revealed that when an axolotl tail is amputated, resident stem cells turn off the genes that are specifically active in neuron-generating cells. In addition, they activate a similar set of genes to that seen in the embryonic cells that form the developing nervous system. These genes speed up cell division and activate an important signaling pathway.

This pathway – the Wnt/PCP pathway – fulfils various developmental roles, one being to orient cell divisions, particularly in elongating tissues. In axolotls, this pathway causes the stem cells to divide equally to increase the number of available stem cells, and orients the direction of these divisions to ensure that the regenerating spinal cord elongates correctly. If this pathway is disrupted, the cells return to dividing unequally, generating nerve cells prematurely and halting the growth of the spinal cord. Such insights could help develop methods of repairing damaged nervous tissue in other animals that cannot regenerate to the extent that axolotls can.

DOI:http://dx.doi.org/10.7554/eLife.10230.002

Fibroblast Growth Factor Receptor-2 Contributes to the Basic Fibroblast Growth Factor-Induced Neuronal Differentiation in Canine Bone Marrow Stromal Cells via Phosphoinositide 3-Kinase/Akt Signaling Pathway

Bone marrow stromal cells (BMSCs) are considered as candidates for regenerative therapy and a useful model for studying neuronal differentiation. The role of basic fibroblast growth factor (bFGF) in neuronal differentiation has been previously studied; however, the signaling pathway involved in this process remains poorly understood. In this study, we investigated the signaling pathway in the bFGF-induced neuronal differentiation of canine BMSCs. bFGF induced the mRNA expression of the neuron marker, microtubule associated protein-2 (MAP2) and the neuron-like morphological change in canine BMSCs. In the presence of inhibitors of fibroblast growth factor receptors (FGFR), phosphatidylinositol 3-kinase (PI3K) and Akt, i.e., SU5402, LY294002, and MK2206, respectively, bFGF failed to induce the MAP2 mRNA expression and the neuron-like morphological change. bFGF induced Akt phosphorylation, but it was attenuated by the FGFR inhibitor SU5402 and the PI3K inhibitor LY294002. In canine BMSCs, expression of FGFR-1 and FGFR-2 was confirmed, but only FGFR-2 activation was detected by cross-linking and immunoprecipitation analysis. Small interfering RNA-mediated knockdown of FGFR-2 in canine BMSCs resulted in the attenuation of bFGF-induced Akt phosphorylation. These results suggest that the FGFR-2/PI3K/Akt signaling pathway is involved in the bFGF-induced neuronal differentiation of canine BMSCs.

Notochord-derived hedgehog is essential for tail regeneration in Xenopus tadpole

Background

Appendage regeneration in amphibians is regulated by the combinatorial actions of signaling molecules. The requirement of molecules secreted from specific tissues is reflected by the observation that the whole process of regeneration can be inhibited if a certain tissue is removed from the amputated stump. Interestingly, urodeles and anurans show different tissue dependencies during tail regeneration. The spinal cord is essential for tail regeneration in urodele but not in anuran larva, whereas the notochord but not the spinal cord is essential for tail regeneration in anuran tadpoles. Sonic hedgehog is one of the signaling molecules responsible for such phenomenon in axolotl, as hedgehog signaling is essential for overall tail regeneration and sonic hedgehog is exclusively expressed in the spinal cord. In order to know whether hedgehog signaling is involved in the molecular mechanism underlying the inconsistent tissue dependency for tail regeneration between anurans and urodeles, we investigated expression of hedgehog signal-related genes in the regenerating tail of Xenopus tadpole and examined the effect of the hedgehog signal inhibitor, cyclopamine, on the tail regeneration.

Results

In Xenopus, sonic hedgehog is expressed exclusively in the notochord but not in the spinal cord of the regenerate. Overall regeneration was severely impaired in cyclopamine-treated tadpoles. Notochord maturation in the regenerate, including cell alignment and vacuolation, and myofiber formation were inhibited. Proliferation of spinal cord cells in the neural ampulla and of mesenchymal cells was also impaired.

Conclusion

As in the axolotl, hedgehog signaling is required for multiple steps in tail regeneration in the Xenopus tadpole, although the location of the Shh source is quite different between the two species. This difference in Shh localization is the likely basis for the differing tissue requirement for tail regeneration between urodeles and anurans.

FGF2 deficit during development leads to specific neuronal cell loss in the enteric nervous system

The largest part of the peripheral nervous system is the enteric nervous system (ENS). It consists of an intricate network of several enteric neuronal subclasses with distinct phenotypes and functions within the gut wall. The generation of these enteric phenotypes is dependent upon appropriate neurotrophic support during development. Glial cell line-derived neurotrophic factor (GDNF) and fibroblast growth factor-2 (FGF2) play an important role in the differentiation and function of the ENS. A lack of GDNF or its receptor (Ret) causes intestinal aganglionosis in mice, while fibroblast growth factor receptor signaling antagonist is identified as regulating proteins in the GDNF/Ret signaling in the developing ENS. Primary myenteric plexus cultures and wholemount preparations of wild type (WT) and FGF2-knockout mice were used to analyze distinct enteric subpopulations. Fractal dimension (D) as a measure of self-similarity is an excellent tool to analyze complex geometric shape and was applied to classify the subclasses of enteric neurons concerning their individual morphology. As a consequence of a detailed analysis of subpopulation variations, wholemount preparations were stained for the calcium binding proteins calbindin and calretinin. The fractal analysis showed a reliable consistence of subgroups with different fractal dimensions (D) in each culture investigated. Seven different neuronal subtypes could be differentiated according to a rising D. Within the same D, the neurite length revealed significant differences between wild type and FGF2-knockout cultures, while the subclass distribution was also altered. Depending on the morphological characteristics, the reduced subgroup was supposed to be a secretomotor neuronal type, which could be confirmed by calbindin and calretinin staining of the wholemount preparations. These revealed a reduction up to 40 % of calbindin-positive neurons in the FGF2-knockout mouse. We therefore consider FGF2 playing a more important role in the fine-tuning of the ENS during development as previously assumed.

Fasciculation and guidance of spinal motor axons in the absence of FGFR2 signaling

During development, fibroblast growth factors (FGF) are essential for early patterning events along the anterior-posterior axis, conferring positional identity to spinal motor neurons by activation of different Hox codes. In the periphery, signaling through one of four fibroblast growth factor receptors supports the development of the skeleton, as well as induction and maintenance of extremities. In previous studies, FGF receptor 2 (FGFR2) was found to interact with axon bound molecules involved in axon fasciculation and extension, thus rendering this receptor an interesting candidate for the promotion of proper peripheral innervation. However, while the involvement of FGFR2 in limb bud induction has been extensively studied, its role during axon elongation and formation of distinct nervous projections has not been addressed so far. We show here that motor neurons in the spinal cord express FGFR2 and other family members during the establishment of motor connections to the forelimb and axial musculature. Employing a conditional genetic approach to selectively ablate FGFR2 from motor neurons we found that the patterning of motor columns and the expression patterns of other FGF receptors and Sema3A in the motor columns of mutant embryos are not altered. In the absence of FGFR2 signaling, pathfinding of motor axons is intact, and also fasciculation, distal advancement of motor nerves and gross morphology and positioning of axonal projections are not altered. Our findings therefore show that FGFR2 is not required cell-autonomously in motor neurons during the formation of initial motor projections towards limb and axial musculature.

Fibroblast growth factor receptor 2 and its role in caudal appendage and craniosynostosis

Caudal appendage is a rare but reported finding seen in association with craniosynostosis. We report a newborn with caudal appendage secondary to sacrococcygeal eversion, a cloverleaf skull, choanal atresia, and a heterozygous mutation of Y375C in the juxtamembrane domain (exon 11) of fibroblast growth factor receptor 2 (FGFR2). Further support of this association are 22 other cases of craniosynostosis with caudal appendage or sacrococcygeal eversion in the literature. Of these, 19 had detectable mutations in FGFR2; 5, the same mutation; and 5, a similar substitution of cysteine for serine. We hypothesize that the association of craniosynostosis and caudal appendage is due to abnormal expression of FGFR2 in the tail bud of a developing embryo based on animal models. Our case and those reported in the literature suggest that in patients with caudal appendage and craniosynostosis, FGRF2 analysis should include regions outside the commonly tested exons 8 and 10, particularly the juxtamembrane domain.

Fibroblast growth factor-2 mRNA expression in the brainstem and spinal cord of normal and chronic spinally transected urodeles

Descending pathways in the spinal cord of adult urodele amphibians show a high regenerative ability after body spinal cord transection; regenerated axons regrow into the transected spinal cord, and hindlimb locomotor recovery occurs spontaneously. Little is currently known about the molecular basis of spinal cord regeneration in urodeles, but it is believed that fibroblast growth factor-2 (FGF2) may play an important role by inducing proliferation of neural progenitor cells. The aim of our study, using in situ hybridization in adult Pleurodeles waltlii, was twofold: 1) to document FGF2 mRNA expression pattern along the brainstem-spinal cord of intact salamanders and 2) to investigate the changes in this pattern in animals unable to display hindlimb locomotor movements and in animals having fully recovered hindlimb locomotor activity after body spinal cord transection. This design establishes a firm basis for further studies on the role of FGF2 in functional recovery of hindlimb locomotion. Our results revealed a decreasing rostrocaudal gradient in FGF2 mRNA expression along the brainstem-spinal cord in intact animals. They further demonstrated a long-lasting up-regulation of FGF2 mRNA expression in response to spinal transection at the midtrunk level, both in brainstem and in the spinal cord below the injury. Finally, double immunolabeling showed that FGF2 was up-regulated in neuroglial, presumably undifferentiated, cells. Therefore, we propose that FGF2 may be involved in cell proliferation and/or neuronal differentiation after body spinal cord transection in salamander and could thus play an important role in functional recovery of locomotion after spinal lesion.

Early gene expression during natural spinal cord regeneration in the salamander Ambystoma mexicanum

In contrast to mammals, salamanders have a remarkable ability to regenerate their spinal cord and recover full movement and function after tail amputation. To identify genes that may be associated with this greater regenerative ability, we designed an oligonucleotide microarray and profiled early gene expression during natural spinal cord regeneration in Ambystoma mexicanum. We sampled tissue at five early time points after tail amputation and identified genes that registered significant changes in mRNA abundance during the first 7 days of regeneration. A list of 1036 statistically significant genes was identified. Additional statistical and fold change criteria were applied to identify a smaller list of 360 genes that were used to describe predominant expression patterns and gene functions. Our results show that a diverse injury response is activated in concert with extracellular matrix remodeling mechanisms during the early acute phase of natural spinal cord regeneration. We also report gene expression similarities and differences between our study and studies that have profiled gene expression after spinal cord injury in rat. Our study illustrates the utility of a salamander model for identifying genes and gene functions that may enhance regenerative ability in mammals.

The Developing Human Spinal Cord Contains Distinct Populations of Neural Precursors

It is becoming apparent that neural stem cells display some differences in their behaviour depending on the region of the CNS they originate from and on whether they are derived from embryonic or adult tissue. Whereas much work has focused on brain neural stem cells, less attention has been paid to spinal cord neural precursors, particularly in the developing human embryo. We briefly review here some of our work which points at some similarities between neural precursors in developing human spinal cords and in animals which can regenerate their spinal cord (e.g. tailed amphibians), and at differences in the properties of human neural precursors with spinal cord development. Altogether these studies suggest the existence of dynamic neural stem cell populations within the developing spinal cord. They also support the notion that thorough characterization of neural stem cells under different culture conditions and analysis of how these may affect their differentiation in vivo after grafting into different injury models is imperative if we are to develop effective cell therapy strategies for spinal cord injury and diseases.

FGFR1 down-regulation in differentiating human brain and spinal cord neurospheres

FGF2 is a key regulator of survival and proliferation of mammalian CNS stem cells. Cells within undifferentiated rodent neurospheres express FGF receptors (FGFRs), but their expression patterns and potential roles in human neurosphere proliferation and differentiation have not been examined. Our aim was to provide an initial overview of the relative profiles of FGFRs before and after differentiation of human neurospheres derived either from embryonic brain or spinal cord. In 'undifferentiated' neurospheres, transcripts from FGFR1 and FGFR2 were consistently detected. FGFR3 could be detected in undifferentiated brain neurospheres and in spinal cord early neurospheres. Following differentiation the most dramatic and consistent change was a decrease in FGFR1 mRNA, suggesting a role for this receptor in maintenance of the undifferentiated state.

Expression of FGF2 in the limb blastema of two Salamandridae correlates with their regenerative capability

Limb regenerative potential in urodeles seems to vary among different species. We observed that Triturus vulgaris meridionalis regenerate their limbs significantly faster than T. carnifex, where a long gap between the time of amputation and blastema formation occurs, and tried to identify cellular and molecular events that may underlie these differences in regenerative capability. Whereas wound healing is comparable in the two species, formation of an apical epidermal cap (AEC), which is required for blastema outgrowth, is delayed for approximately three weeks in T. carnifex. Furthermore, fewer nerve fibres are present distally early after amputation, consistent with the late onset of blastemal cell proliferation observed in T. carnifex. We investigated whether different expression of putative blastema mitogens, such as FGF1 and FGF2, in these species may underlie differences in the progression of regeneration. We found that whereas FGF1 is detected in the epidermis throughout the regenerative process, FGF2 onset of expression in the wound epidermis of both species coincides with AEC formation and initiation of blastemal cell proliferation, which is delayed in T. carnifex, and declines thereafter. In vitro studies showed that FGF2 activates MCM3, a factor essential for DNA replication licensing activity, and can be produced by blastemal cells themselves, indicating an autocrine action. These results suggest that FGF2 plays a key role in the initiation of blastema growth.

Changes in spinal cord regenerative ability through phylogenesis and development: lessons to be learnt

Lower vertebrates, such as fish and amphibians, and developing higher vertebrates can regenerate complex body structures, including significant portions of their central nervous system. It is still poorly understood why this potential is lost with evolution and development and becomes very limited in adult mammals. In this review, we will discuss the current knowledge on the cellular and molecular changes after spinal cord injury in adult tailed amphibians, where regeneration does take place, and in developing chick and mammalian embryos at different developmental stages. We will focus on the recruitment of progenitor cells to repair the damage and discuss possible roles of changes in early response to injury, such as cell death by apoptosis, and of myelin-associated proteins, such as Nogo, in the transition between regeneration-competent and regeneration-incompetent stages of development. A better understanding of the mechanisms underlying spontaneous regeneration of the spinal cord in vivo in amphibians and in the chick embryo will help to devise strategies for restoring function to damaged or diseased nervous tissues in mammals.