Subtypes of hypoxia-responsive cells differentiate into neurons in spinal cord of zebrafish embryos after hypoxic stress

The Promising Role of a Zebrafish Model Employed in Neural Regeneration Following a Spinal Cord Injury

Spinal cord injury (SCI) is a devastating event that results in a wide range of physical impairments and disabilities. Despite the advances in our understanding of the biological response to injured tissue, no effective treatments are available for SCIs at present. Some studies have addressed this issue by exploring the potential of cell transplantation therapy. However, because of the abnormal microenvironment in injured tissue, the survival rate of transplanted cells is often low, thus limiting the efficacy of such treatments. Many studies have attempted to overcome these obstacles using a variety of cell types and animal models. Recent studies have shown the utility of zebrafish as a model of neural regeneration following SCIs, including the proliferation and migration of various cell types and the involvement of various progenitor cells. In this review, we discuss some of the current challenges in SCI research, including the accurate identification of cell types involved in neural regeneration, the adverse microenvironment created by SCIs, attenuated immune responses that inhibit nerve regeneration, and glial scar formation that prevents axonal regeneration. More in-depth studies are needed to fully understand the neural regeneration mechanisms, proteins, and signaling pathways involved in the complex interactions between the SCI microenvironment and transplanted cells in non-mammals, particularly in the zebrafish model, which could, in turn, lead to new therapeutic approaches to treat SCIs in humans and other mammals.

Macrophage–Neuroglia Interactions in Promoting Neuronal Regeneration in Zebrafish

The human nervous system exhibits limited regenerative capabilities following damage to the central nervous system (CNS), leading to a scarcity of effective treatments for nerve function recovery. In contrast, zebrafish demonstrate remarkable regenerative abilities, making them an ideal model for studying the modulation of inflammatory processes after injury. Such research holds significant translational potential to enhance our understanding of recovery from damage and disease. Macrophages play a crucial role in tissue repair and regeneration, with their subpopulations indirectly promoting axonal regeneration through developmental signals. The AP-1 signaling pathway, mediated by TNF/Tnfrsf1a, can elevate HDAC1 expression and facilitate regeneration. Furthermore, following spinal cord injury (SCI), pMN progenitors have been observed to switch between oligodendrocyte and motor neuron fates, with macrophage-secreted TNF-α potentially regulating the differentiation of ependymal–radial glia progenitors and oligodendrocytes. Radial glial cells (RGs) are also essential for CNS regeneration in zebrafish, as they perform neurogenesis and gliogenesis, with specific RG subpopulations potentially existing for the generation of neurons and oligodendrocytes. This review article underscores the critical role of macrophages and their subpopulations in tissue repair and regeneration, focusing on their secretion of TNF-α, which promotes axonal regeneration in zebrafish. We also offer insights into the molecular mechanisms underlying TNF-α’s ability to facilitate axonal regeneration and explore the potential of pMN progenitor cells and RGs following SCI in zebrafish. The review concludes with a discussion of various unresolved questions in the field, and ideas are suggested for future research. Studying innate immune cell interactions with neuroglia following injury may lead to the development of novel strategies for treating the inflammatory processes associated with regenerative medicine, which are commonly observed in injury and disease.

Genomic Structure, Protein Character, Phylogenic Implication, and Embryonic Expression Pattern of a Zebrafish New Member of Zinc Finger BED-Type Gene Family

We reported a new member of the C₂H₂-zinc-finger BED-type (ZBED) protein family found in zebrafish (Danio rerio). It was previously assigned as an uncharacterized protein LOC569044 encoded by the Zgc:161969 gene, the transcripts of which were highly expressed in the CNS after the spinal cord injury of zebrafish. As such, this novel gene deserves a more detailed investigation. The 2.79-kb Zgc:161969 gene contains one intron located on Chromosome 6 at 16,468,776-16,475,879 in the zebrafish genome encoding a 630-aa protein LOC569044. This protein is composed of a DNA-binding BED domain, which is highly conserved among the ZBED protein family, and a catalytic domain consisting of an α-helix structure and an hAT dimerization region. Phylogenetic analysis revealed the LOC569044 protein to be clustered into the monophyletic clade of the ZBED protein family of golden fish. Specifically, the LOC569044 protein was classified as closely related to the monophyletic clades of zebrafish ZBED4-like isoforms and ZBED isoform 2. Furthermore, Zgc:161969 transcripts represented maternal inheritance, expressed in the brain and eyes at early developmental stages and in the telencephalon ventricular zone at late developmental stages. After characterizing the LOC569044 protein encoded by the Zgc:161969 gene, it was identified as a new member of the zebrafish ZBED protein family, named the ZBEDX protein.

Anp32a Promotes Neuronal Regeneration after Spinal Cord Injury of Zebrafish Embryos

After spinal cord injury (SCI) in mammals, neuronal regeneration is limited; in contrast, such regeneration occurs quickly in zebrafish. Member A of the acidic nuclear phosphoprotein 32 (ANP32a) family is involved in neuronal development, but its function is controversial, and its involvement in zebrafish SCI remains unknown. To determine the role of zebrafish ANP32a in the neuronal regeneration of SCI embryos, we microinjected ANP32a mRNA into embryos from zebrafish transgenic line Tg(mnx1:GFP) prior to SCI. Compared to control SCI embryos, the results showed that the regeneration of spinal cord and resumption of swimming capability were promoted by the overexpression of ANP32a mRNA but reduced by its knockdown. We next combined fluorescence-activated cell sorting with immunochemical staining of anti-GFAP and immunofluorescence staining against anti-PH3 on Tg(gfap:GFP) SCI embryos. The results showed that ANP32a promoted the proliferation and cell number of radial glial cells at the injury epicenter at 24 h post-injury (hpi). Moreover, when we applied BrdU labeling to SCI embryos derived from crossing the Tg(gfap:GFP) and Tg(mnx1:TagRFP) lines, we found that both radial glial cells and motor neurons had proliferated, along with their increased cell numbers in Anp32a-overexpression SCI-embryos. On this basis, we conclude that ANP32a plays a positive role in the regeneration of zebrafish SCI embryos.

Hypoxia-Responsive Subtype Cells Differentiate Into Neurons in the Brain of Zebrafish Embryos Exposed to Hypoxic Stress

Severe hypoxia results in complete loss of central nervous system (CNS) function in mammals, while several other vertebrates, such as zebrafish, can regenerate after hypoxia-induced injury of CNS. Since the cellular mechanism involved in this remarkable feature of other vertebrates is still unclear, we studied the cellular regeneration of zebrafish brain, employing zebrafish embryos from transgenic line huORFZ exposed to hypoxia and then oxygen recovery. GFP-expressing cells, identified in some cells of the CNS, including some brain cells, were termed as hypoxia-responsive recovering cells (HrRCs). After hypoxia, HrRCs did not undergo apoptosis, while most non-GFP-expressing cells, including neurons, did. Major cell types of HrRCs found in the brain of zebrafish embryos induced by hypoxic stress were neural stem/progenitor cells (NSPCs) and radial glia cells (RGs), that is, subtypes of NSPCs (NSPCs-HrRCs) and RGs (RGs-HrRCs) that were induced by and sensitively responded to hypoxic stress. Interestingly, among HrRCs, subtypes of NSPCs- or RGs-HrRCs could proliferate and differentiate into early neurons during oxygen recovery, suggesting that these subtype cells might play a critical role in brain regeneration of zebrafish embryos after hypoxic stress.

Poly(U)-specific endoribonuclease ENDOU promotes translation of human CHOP mRNA by releasing uORF element-mediated inhibition

Upstream open reading frames (uORFs) are known to negatively affect translation of the downstream ORF. The regulatory proteins involved in relieving this inhibition are however poorly characterized. In response to cellular stress, eIF2α phosphorylation leads to an inhibition of global protein synthesis, while translation of specific factors such as CHOP is induced. We analyzed a 105‐nt inhibitory uORF in the transcript of human CHOP (huORF^chop) and found that overexpression of the zebrafish or human ENDOU poly(U)‐endoribonuclease (Endouc or ENDOU‐1, respectively) increases CHOP mRNA translation also in the absence of stress. We also found that Endouc/ENDOU‐1 binds and cleaves the huORF^chop transcript at position 80G‐81U, which induces CHOP translation independently of phosphorylated eIF2α. However, both ENDOU and phospho‐eIF2α are nonetheless required for maximal translation of CHOP mRNA. Increased levels of ENDOU shift a huORF^chop reporter as well as endogenous CHOP transcripts from the monosome to polysome fraction, indicating an increase in translation. Furthermore, we found that the uncapped truncated huORF^chop‐69‐105‐nt transcript contains an internal ribosome entry site (IRES), facilitating translation of the cleaved transcript. Therefore, we propose a model where ENDOU‐mediated transcript cleavage positively regulates CHOP translation resulting in increased CHOP protein levels upon stress. Specifically, CHOP transcript cleavage changes the configuration of huORF^chop thereby releasing its inhibition and allowing the stalled ribosomes to resume translation of the downstream ORF.

Injury-induced Cavl-expressing cells at lesion rostral side play major roles in spinal cord regeneration

The extent of cellular heterogeneity involved in neuronal regeneration after spinal cord injury (SCI) remains unclear. Therefore, we established stress-responsive transgenic zebrafish embryos with SCI. As a result, we found an SCI-induced cell population, termed SCI stress-responsive regenerating cells (SrRCs), essential for neuronal regeneration post-SCI. SrRCs were mostly composed of subtypes of radial glia (RGs-SrRCs) and neuron stem/progenitor cells (NSPCs-SrRCs) that are able to differentiate into neurons, and they formed a bridge across the lesion and connected with neighbouring undamaged motor neurons post-SCI. Compared to SrRCs at the caudal side of the SCI site (caudal-SrRCs), rostral-SrRCs participated more actively in neuronal regeneration. After RNA-seq analysis, we discovered that caveolin 1 (cav1) was significantly upregulated in rostral-SrRCs and that cav1 was responsible for the axonal regrowth and regenerative capability of rostral-SrRCs. Collectively, we define a specific SCI-induced cell population, SrRCs, involved in neuronal regeneration, demonstrate that rostral-SrRCs exhibit higher neuronal differentiation capability and prove that cav1 is predominantly expressed in rostral-SrRCs, playing a major role in neuronal regeneration after SCI.

The Neuronal Regeneration of Adult Zebrafish After Spinal Cord Injury Is Enhanced by Transplanting Optimized Number of Neural Progenitor Cells

Cell transplantation is commonly used to study the regeneration and repair of the nervous system in animals. However, a technical platform used to evaluate the optimum number of transplanted cells in the recipient’s spinal cord is little reported. Therefore, to develop such platform, we used a zebrafish model, which has transparent embryos, and transgenic line huORFZ, which generates green fluorescent protein (GFP)-expressing cells in the central nervous system under hypoxic stress. After GFP-expressing cells, also termed as hypoxia-responsive recovering cells, were obtained from hypoxia-exposed huORFZ embryos, we transplanted these GFP-(+) cells into the site of spinal cord injury (SCI) in adult wild-type zebrafish, followed by assessing the relationship between number of transplanted cells and the survival rate of recipients. When 100, 300, 500, and 1,000 GFP-(+) donor cells were transplanted into the lesion site of SCI-treated recipients, we found that recipient adult zebrafish transplanted with 300 donor cells had the highest survival rate. Those GFP-(+) donor cells could undergo proliferation and differentiation into neuron in recipients. Furthermore, transplantation of GFP-(+) cells into adult zebrafish treated with SCI was able to enhance the neuronal regeneration of recipients. In contrast, those fish transplanted with over 500 cells showed signs of inflammation around the SCI site, resulting in higher mortality. In this study, we developed a technological platform for transplanting cells into the lesion site of SCI-treated adult zebrafish and defined the optimum number of successfully transplanted cells into recipients, as 300, and those GFP-(+) donor cells could enhance recipient’s spinal cord regeneration. Thus, we provided a practical methodology for studying cell transplantation therapy in neuronal regeneration of zebrafish after SCI.

The second pharyngeal pouch is generated by dynamic remodeling of endodermal epithelium in zebrafish

Pharyngeal arches (PAs) are segmented by endodermal outpocketings called pharyngeal pouches (PPs). Anterior and posterior PAs appear to be generated by different mechanisms, but it is unclear how the anterior and posterior PAs combine. Here, we addressed this issue with precise live imaging of PP development and cell tracing of pharyngeal endoderm in zebrafish embryos. We found that two endodermal bulges are initially generated in the future second PP (PP2) region, which separates anterior and posterior PAs. Subsequently, epithelial remodeling causes contact between these two bulges, resulting in the formation of mature PP2 with a bilayered morphology. The rostral and caudal bulges develop into the operculum and gill, respectively. Development of the caudal PP2 and more posterior PPs is affected by impaired retinoic acid signaling or pax1a/b dysfunction, suggesting that the rostral front of posterior PA development corresponds to the caudal PP2. Our study clarifies an aspect of PA development that is essential for generation of a seamless array of PAs in zebrafish.

A new member of the forkhead box protein family in zebrafish: Domain composition, phylogenetic implication and embryonic expression pattern

In this study, we reported a novel member of Forkhead box (Fox) proteins found in model zebrafish (Danio rerio). This new gene we cloned was primarily assigned as zgc:162612, which locates on Chromosome 3 at 26,108,033-26,109,322 in the zebrafish genome, but encodes an uncharacterized protein, LOC100037333. After we determined the nucleotide and deduced amino acid sequences of zgc:162612, we found that zgc:162612 is an intronless gene and contains 1290 base pairs encoding 308 amino acid residues. Zgc:162612 protein is composed of a highly conserved DNA-binding domain similar to that of the Fox protein family, but with variable terminal domains. Based on phylogenetic analysis of all known members within the zebrafish Fox protein family, zgc:162612 was clustered into the zebrafish FoxD isoform subfamily. Thus, we confirmed zgc:162612 as the zebrafish FoxD7 gene. The deduced amino acid sequence of zebrafish FoxD7 shared 49, 49, 74, 63 and 74% identities with that of zebrafish FoxD1, FoxD2, FoxD3, FoxD4 and FoxD6, respectively. Compared with all known FoxD proteins in invertebrate and vertebrate species, the zebrafish FoxD7 is categorized in the same monophyletic group along with FoxD of cephalochordate and sea urchin. Whole-mount in situ hybridization demonstrated that zebrafish FoxD7 transcripts represented maternal inheritance and were ubiquitously expressed throughout the whole embryo at 12hpf. Moreover, while FoxD7 transcripts were expressed in the brain, spinal cord, fins and eyes at early developmental stages, they were mainly presented in the telencephalon ventricular zone at late developmental stages, suggesting that FoxD7 may play roles in neurogenesis and organogenesis during development of zebrafish. Taken together, we have defined a previously uncharacterized gene in the zebrafish genome, zgc:162612, and revealed that Zgc:162612 encodes a novel putative transcription factor, thus becoming a new member of the zebrafish FoxD isoform subfamily.

Infrared Laser-Mediated Gene Induction at the Single-Cell Level in the Regenerating Tail of Xenopus laevis Tadpoles

We describe a precise and reproducible gene-induction method in the amphibian, Xenopus laevis Tetrapod amphibians are excellent models for studying the mechanisms of three-dimensional organ regeneration because they have an exceptionally high regenerative ability. However, spatial and temporal manipulation of gene expression has been difficult in amphibians, hindering studies on the molecular mechanisms of organ regeneration. Recently, however, development of a Xenopus transgenic system with a heat-shock-inducible gene has enabled the manipulation of specific genes. Here, we applied an infrared laser-evoked gene operator (IR-LEGO) system to the regenerating tail of Xenopus tadpoles. In this method, a local heat shock by laser irradiation induces gene expression at the single-cell level. After amputation, Xenopus tadpoles regenerate a functional tail, including spinal cord. The regenerating tail is flat and transparent enabling the targeting of individual cells by laser irradiation. In this protocol, a single neural progenitor cell in the spinal cord of the regenerating tail is labeled with heat-shock-inducible green fluorescent protein (GFP). Gene induction at the single-cell level provides a method for rigorous cell-lineage tracing and for analyzing gene function in both cell-autonomous and noncell-autonomous contexts. The method can be modified to study the regeneration of limbs or organs in other amphibians, including Xenopus tropicalis, newts, and salamanders.

Redox regulation in regenerative medicine and tissue engineering: The paradox of oxygen

One of the biggest challenges in tissue engineering and regenerative medicine is to incorporate a functioning vasculature to overcome the consequences of a lack of oxygen and nutrients in the tissue construct. Otherwise, decreased oxygen tension leads to incomplete metabolism and the formation of the so‐called reactive oxygen species (ROS). Cells have many endogenous antioxidant systems to ensure a balance between ROS and antioxidants, but if this balance is disrupted by factors such as high levels of ROS due to long‐term hypoxia, there will be tissue damage and dysfunction. Current attempts to solve the oxygen problem in the field rarely take into account the importance of the redox balance and are instead centred on releasing or generating oxygen. The first problem with this approach is that although oxygen is necessary for life, it is paradoxically also a highly toxic molecule. Furthermore, although some oxygen‐generating biomaterials produce oxygen, they also generate hydrogen peroxide, a ROS, as an intermediate product. In this review, we discuss why it would be a superior strategy to supplement oxygen delivery with molecules to safeguard the important redox balance. Redox sensor proteins that can stimulate the anaerobic metabolism, angiogenesis, and enhancement of endogenous antioxidant systems are discussed as promising targets. We propose that redox regulating biomaterials have the potential to tackle some of the challenges related to angiogenesis and that the knowledge in this review will help scientists in tissue engineering and regenerative medicine realize this aim.