Dynamic cell interactions allow spinal cord regeneration in zebrafish

Dopamine Receptors Gene Expression Pattern and Locomotor Improvement Differ Between Female and Male Zebrafish During Spinal Cord Auto Repair

The dopaminergic system, a spinal cord (SC) motor circuit regulator, is administrated by sexual hormones and evolutionary conserved in all vertebrates. Accordingly, we hypothesized that the dopamine receptor (DAR) expression pattern may be dissimilar in female and male zebrafish SC auto repair. We implemented an uncomplicated method to induce spinal cord injury (SCI) on fully reproductive adult zebrafish, in both genders. SCI was induced using a 28-gauge needle at 9th-10th vertebra without skin incision. Thereupon, lesioned SC was harvested for DAR gene expression analysis; zebrafish were tracked routinely for any improvement in swim distance, speed, and their roaming capabilities/preference. Our findings revealed discrepancies between drd2a, drd2b, drd3, drd4a, and drd4b expression patterns at 1, 7, and 14 days postinjury (DPI) between female and male zebrafish. The receptors were mostly upregulated at 7 DPI in both genders, whereas drd2a and drd2b were mostly maximized in females. Surprisingly, drd3 was measured greater even in intact SC in males. In addition, female zebrafish were able to swim farther distances more accelerated, in multiple directions, by engaging more caudal muscles compared with males, of course with no statistical significance. Indeed, females were able to generate whole-body rotation and move forward using the muscles downstream to the lesion site, whereas the coordinated movement in males was accomplished by rostral muscles. In conclusion, there are differences in DAR gene expression pattern throughout SC autonomous recovery between adult female and male zebrafish, and also, female locomotion seems to ameliorate more rapidly.

Zebrafish Fin: Complex Molecular Interactions and Cellular Mechanisms Guiding Regeneration

The zebrafish caudal fin has become a popular model to study cellular and molecular mechanisms of regeneration due to its high regenerative capacity, accessibility for experimental manipulations, and relatively simple anatomy. The formation of a regenerative epidermis and blastema are crucial initial events and tightly regulated. Both the regenerative epidermis and the blastema are highly organized structures containing distinct domains, and several signaling pathways regulate the formation and interaction of these domains. Bone is the major tissue regenerated from the progenitor cells of the blastema. Several cellular mechanisms can provide source cells for blastemal (pre-)osteoblasts, including dedifferentiation of differentiated osteoblasts and de novo formation from other cell types, providing intriguing examples of cellular plasticity. In recent years, omics analyses and single-cell approaches have elucidated genetic and epigenetic regulation, increasing our knowledge of the surprisingly complex coordination of various mechanisms to achieve successful restoration of a seemingly simple structure.

Molecular and Cellular Analysis of the Repair of Zebrafish Optic Tectum Meninges Following Laser Injury

We studied cell recruitment following optic tectum (OT) injury in zebrafish (Danio rerio), which has a remarkable ability to regenerate many of its organs, including the brain. The OT is the largest dorsal layered structure in the zebrafish brain. In juveniles, it is an ideal structure for imaging and dissection. We investigated the recruited cells within the juvenile OT during regeneration in a Pdgfrβ-Gal4:UAS-EGFP line in which pericytes, vascular, circulating, and meningeal cells are labeled, together with neurons and progenitors. We first performed high-resolution confocal microscopy and single-cell RNA-sequencing (scRNAseq) on EGFP-positive cells. We then tested three types of injury with very different outcomes (needle (mean depth in the OT of 200 µm); deep-laser (depth: 100 to 200 µm depth); surface-laser (depth: 0 to 100 µm)). Laser had the additional advantage of better mimicking of ischemic cerebral accidents. No massive recruitment of EGFP-positive cells was observed following laser injury deep in the OT. This type of injury does not perturb the meninx/brain–blood barrier (BBB). We also performed laser injuries at the surface of the OT, which in contrast create a breach in the meninges. Surprisingly, one day after such injury, we observed the migration to the injury site of various EGFP-positive cell types at the surface of the OT. The migrating cells included midline roof cells, which activated the PI3K-AKT pathway; fibroblast-like cells expressing numerous collagen genes and most prominently in 3D imaging; and a large number of arachnoid cells that probably migrate to the injury site through the activation of cilia motility genes, most likely being direct targets of the FOXJ1a gene. This study, combining high-content imaging and scRNAseq in physiological and pathological conditions, sheds light on meninges repair mechanisms in zebrafish that probably also operate in mammalian meninges.

Hippo-Yap/Taz signalling in zebrafish regeneration

The extent of tissue regeneration varies widely between species. Mammals have a limited regenerative capacity whilst lower vertebrates such as the zebrafish (Danio rerio), a freshwater teleost, can robustly regenerate a range of tissues, including the spinal cord, heart, and fin. The molecular and cellular basis of this altered response is one of intense investigation. In this review, we summarise the current understanding of the association between zebrafish regeneration and Hippo pathway function, a phosphorylation cascade that regulates cell proliferation, mechanotransduction, stem cell fate, and tumorigenesis, amongst others. We also compare this function to Hippo pathway activity in the regenerative response of other species. We find that the Hippo pathway effectors Yap/Taz facilitate zebrafish regeneration and that this appears to be latent in mammals, suggesting that therapeutically promoting precise and temporal YAP/TAZ signalling in humans may enhance regeneration and hence reduce morbidity.

MicroRNA roles in regeneration: Multiple lessons from zebrafish

MicroRNAs (miRNAs) are small noncoding RNAs with pivotal roles in the control of gene expression. By comparing the miRNA profiles of uninjured vs. regenerating tissues and structures, several studies have found that miRNAs are potentially involved in the regenerative process. By inducing miRNA overexpression or inhibition, elegant experiments have directed regenerative responses validating relevant miRNA-to-target interactions. The zebrafish (Danio rerio) has been the epicenter of regenerative research because of its exceptional capability to self-repair damaged tissues and body structures. In this review, we discuss recent discoveries that have improved our understanding of the impact of gene regulation mediated by miRNAs in the context of the regeneration of fins, heart, retina, and nervous tissue in zebrafish. We compiled what is known about the miRNA control of regeneration in these tissues and investigated the links among up-regulated and down-regulated miRNAs, their putative or validated targets, and the regenerative process. Finally, we briefly discuss the forthcoming prospects, highlighting directions and the potential for further development of this field.

The Structure of the Spinal Cord Ependymal Region in Adult Humans Is a Distinctive Trait among Mammals

In species that regenerate the injured spinal cord, the ependymal region is a source of new cells and a prominent coordinator of regeneration. In mammals, cells at the ependymal region proliferate in normal conditions and react after injury, but in humans, the central canal is lost in the majority of individuals from early childhood. It is replaced by a structure that does not proliferate after damage and is formed by large accumulations of ependymal cells, strong astrogliosis and perivascular pseudo-rosettes. We inform here of two additional mammals that lose the central canal during their lifetime: the Naked Mole-Rat (NMR, Heterocephalus glaber) and the mutant hyh (hydrocephalus with hop gait) mice. The morphological study of their spinal cords shows that the tissue substituting the central canal is not similar to that found in humans. In both NMR and hyh mice, the central canal is replaced by tissue reminiscent of normal lamina X and may include small groups of ependymal cells in the midline, partially resembling specific domains of the former canal. However, no features of the adult human ependymal remnant are found, suggesting that this structure is a specific human trait. In order to shed some more light on the mechanism of human central canal closure, we provide new data suggesting that canal patency is lost by delamination of the ependymal epithelium, in a process that includes apical polarity loss and the expression of signaling mediators involved in epithelial to mesenchymal transitions.

A unique macrophage subpopulation signals directly to progenitor cells to promote regenerative neurogenesis in the zebrafish spinal cord

Central nervous system injury re-initiates neurogenesis in anamniotes (amphibians and fishes), but not in mammals. Activation of the innate immune system promotes regenerative neurogenesis, but it is fundamentally unknown whether this is indirect through the activation of known developmental signaling pathways or whether immune cells directly signal to progenitor cells using mechanisms that are unique to regeneration. Using single-cell RNA-seq of progenitor cells and macrophages, as well as cell-type-specific manipulations, we provide evidence for a direct signaling axis from specific lesion-activated macrophages to spinal progenitor cells to promote regenerative neurogenesis in zebrafish. Mechanistically, TNFa from pro-regenerative macrophages induces Tnfrsf1a-mediated AP-1 activity in progenitors to increase regeneration-promoting expression of hdac1 and neurogenesis. This establishes the principle that macrophages directly communicate to spinal progenitor cells via non-developmental signals after injury, providing potential targets for future interventions in the regeneration-deficient spinal cord of mammals.

Neural circuit reorganisation after spinal cord injury in zebrafish

Spinal cord injuries disrupt signalling from the brain leading to loss of limb, locomotion, sexual and bladder function, usually irreversible in humans. In zebrafish, recovery of function occurs in a few days for larvae or a few weeks for adults due to regrowth of axons and de novo neurogenesis. Together with its genetic amenability and optical clarity, this makes zebrafish a powerful animal model to study circuit reorganisation after spinal cord injuries. With the fast evolution of techniques, we can forecast significative improvements of our knowledge of the mechanisms leading to successful or failed recovery of spinal cord function. We review here the present knowledge on the subject, the new technological approaches and we propose future directions of research.