Characterization of Pax2 expression in the goldfish optic nerve head during retina regeneration

Oligodendrocyte origin and development in the zebrafish visual system

Oligodendrocytes are the myelinating cells in the central nervous system. In birds and mammals, the oligodendrocyte progenitor cells (OPCs) originate in the preoptic area (POA) of the hypothalamus. However, it remains unclear in other vertebrates such as fish. Thus, we have studied the early progression of OPCs during zebrafish visual morphogenesis from 2 days post fertilization (dpf) until 11 dpf using the olig2:EGFP transgenic line; and we have analyzed the differential expression of transcription factors involved in oligodendrocyte differentiation: Sox2 (using immunohistochemistry) and Sox10 (using the transgenic line sox10:tagRFP). The first OPCs (olig2:EGFP/Sox2) were found at 2 dpf in the POA. From 3 dpf onwards, these olig2:EGFP/Sox2 cells migrate to the optic chiasm, where they invade the optic nerve (ON), extending toward the retina. At 5 dpf, olig2:EGFP/Sox2 cells in the ON also colocalize with sox10:tagRFP. When olig2:EGFP cells differentiate and present more projections, they become positive only for sox10:tagRFP. olig2:EGFP/sox10: tagRFP cells ensheath the ON by 5 dpf when they also become positive for a myelin marker, based on the mbpa:tagRFPt transgenic line. We also found olig2:EGFP cells in other regions of the visual system. In the central retina at 2 dpf, they are positive for Sox2 but later become restricted to the proliferative germinal zone without this marker. In the ventricular areas of the optic tectum, olig2:EGFP cells present Sox2 but arborized ones sox10:tagRFP instead. Our data matches with other models, where OPCs are specified in the POA and migrate to the ON through the optic chiasm.

Cultures of glial cells from optic nerve of two adult teleost fish: Astatotilapia burtoni and Danio rerio

BACKGROUND

In vitro studies are very useful to increase the knowledge of different cell types and could be the key to understand cell metabolism and function. Fish optic nerves (ON) can recover visual functions by reestablishing its structure and reconnecting the axons of ganglion cells. This is because fish show spontaneous regeneration of the central nervous system which does not occur in mammals. In addition, several studies have indicated that glial cells of ON have different properties in comparison to the glial cells from brain or retina. Consequently, providing an in vitro tool will be highly beneficial to increase the knowledge of these cells.

NEW METHOD

We developed a cell culture protocol to isolate glial cells from ON of two teleost fish species, Danio rerio and Astatotilapia burtoni.

RESULTS

The optimized protocol allowed us to obtain ON cells and brain-derived cells from adult teleost fish. These cells were characterized as glial cells and their proprieties in vitro were analyzed.Comparison with Existing Method(s): Although it is striking that ON glial cells show peculiarities, their study in vitro has been limited by the only published protocol going back to the 1990s. Our protocol makes glial cells of different fish species available for experiments and studies to increase the understanding of these glial cell types.

CONCLUSIONS

This validated and effective in vitro tool increases the possibilities on studies of glial cells from fish ON which implies a reduction in animal experimentation.

GFAP expression in the optic nerve and increased H2S generation in the integration centers of the rainbow trout (Oncorhynchus mykiss) brain after unilateral eye injury

Hydrogen sulfide (H₂S) is considered as a protective factor against cardiovascular disorders. However, there are few reports on the effects of H₂S in the central nervous system during stress or injury. Previous studies on goldfish have shown that astrocytic response occurs in the damaged and contralateral optic nerves. Glial fibrillary acidic protein (GFAP) concentration in the optic nerves of rainbow trout has not been measured previously. This study further characterized the astrocytic response in the optic nerve and the brain of a rainbow trout (Oncorhynchus mykiss) after unilateral eye injury and estimated the amount of H₂S-producing enzyme cystathionine β-synthase (CBS) in the brain of the rainbow trout. Within 1 week after unilateral eye injury, a protein band corresponding to a molecular weight of 50 kDa was identified in the ipsi- and contralateral optic nerves of the rainbow trout. The concentration of GFAP in the injured optic nerve increased compared to the protein concentration on the contralateral side. The results of a quantitative analysis of GFAP⁺ cell distribution in the contralateral optic nerve showed the largest number of GFAP⁺ cells and fibers in the optic nerve head. In the damaged optic nerve, patterns of GFAP⁺ cell migration and large GFAP⁺ bipolar activated astrocytes were detected at 1 week after unilateral eye injury. The study of H₂S-producing system after unilateral eye injury in the rainbow trout was conducted using enzyme-linked immunosorbent assay, western blot analysis, and immunohistochemistry of polyclonal antibodies against CBS in the integrative centers of the brain: telencephalon, optic tectum, and cerebellum. Enzyme-linked immunosorbent assay results showed a 1.7-fold increase in CBS expression in the rainbow trout brain at 1 week after unilateral eye injury compared with that in intact animals. In the ventricular and subventricular regions of the rainbow trout telencephalon, CBS⁺ radial glia and neuroepithelial cells were identified. After unilateral eye injury, the number of CBS⁺ neuroepithelial cells in the pallial and subpallial periventricular regions of the telencephalon increased. In the optic tectum, unilateral eye injury led to an increase in CBS expression in radial glial cells; simultaneously, the number of CBS⁺ neuroepithelial cells decreased in intact animals. In the cerebellum of the rainbow trout, neuroglial interrelationships were revealed, where H₂S was released, apparently, from astrocyte-like cells. The organization of H₂S-producing cell complexes suggests that, the amount of glutamate produced in the rainbow trout cerebellum and its reuptake was controlled by astrocyte-like cells, reducing its excitotoxicity. In the dorsal matrix zone and granular eminences of the rainbow trout cerebellum, CBS was expressed in neuroepithelial cells. After unilateral eye injury, the level of CBS activity increased in all parts of the cerebellum. An increase in the number of H₂S-producing cells was a response to oxidative stress after unilateral eye injury, and the overproduction of H₂S in the cerebellum occurred to neutralize reactive oxygen species, providing the cells of the rainbow trout cerebellum with a protective effect. A structural reorganization in the dorsal matrix zone, associated with the appearance of an additional CBS⁺ apical zone, and a decrease in the enzyme activity in the dorsal matrix zone, was revealed in the zones of constitutive neurogenesis. All experiments were approved by the Commission on Biomedical Ethics, A.V. Zhirmunsky National Scientific Center of Marine Biology (NSCMB), Far Eastern Branch, Russian Academy of Science (FEB RAS) (approval No. 1) on July 31, 2019.

Model systems for regeneration: zebrafish

Tissue damage can resolve completely through healing and regeneration, or can produce permanent scarring and loss of function. The response to tissue damage varies across tissues and between species. Determining the natural mechanisms behind regeneration in model organisms that regenerate well can help us develop strategies for tissue recovery in species with poor regenerative capacity (such as humans). The zebrafish (Danio rerio) is one of the most accessible vertebrate models to study regeneration. In this Primer, we highlight the tools available to study regeneration in the zebrafish, provide an overview of the mechanisms underlying regeneration in this system and discuss future perspectives for the field.

Transcriptomic responses of regenerating earthworms (Eisenia foetida) to retinoic acid reveals the role of pluripotency genes

Exogenous retinoic acid (RA) delays and disturbs the regeneration of Eisenia foetida and inhibits the expression of pluripotent gene Sox2. However, studies of E. foetida conducted at the molecular level have been unable to elucidate its regeneration and mechanisms of RA effects on its regeneration. We merged existing transcriptomic data for E. foetida to generate a high-confidence set of transcriptomes. The de novo assembly of transcriptomes was performed by using the Trinity method, and functional annotations were analysed. We performed RNA-seq on four samples of regenerating tail fragments, three across a time-course (0, 3 and 7 days post amputation) and the fourth sample exposed to RA (7 days post amputation). E. foetida regeneration genes underwent significant upregulation and downregulation over the examined time periods, which may have been caused by a shared regulatory programme controlled by multiple gene families. The inhibition of RA against earthworm regeneration is likely related to the expression of these genes. Using annotation data and clustering, we also identified specific transcripts of 6 gene superfamilies enriched among genes exhibiting differential expression during regeneration periods and exhibiting the same expression patterns as those of the Sox2 gene. The regeneration transcriptome of tail fragment regeneration serves as a strong resource for investigating global expression changes that occur during regeneration and the toxicity of RA. This study offers insight for better understanding the regeneration of lower animals and molecular mechanisms of RA toxicity in invertebrates.

Neurolin expression in the optic nerve and immunoreactivity of Pax6-positive niches in the brain of rainbow trout (Oncorhynchus mykiss) after unilateral eye injury

In contrast to astrocytes in mammals, fish astrocytes promote axon regeneration after brain injury and actively participate in the regeneration process. Neurolin, a regeneration-associated, Zn8-labeled protein, is involved in the repair of damaged optic nerve in goldfish. At 1 week after unilateral eye injury, the expression of neurolin in the optic nerve and chiasm, and the expression of Pax6 that influences nervous system development in various brain regions in the rainbow trout (Oncorhynchus mykiss) were detected. Immunohistochemical staining revealed that the number of Zn8⁺ cells in the optic nerve head and intraorbital segment was obviously increased, and the increase in Zn8⁺ cells was also observed in the proximal and distal parts of injured optic nerve. This suggests that Zn8⁺ astrocytes participate in optic nerve regeneration. ELISA results revealed that Pax6 protein increased obviously at 1 week post-injury. Immunohistochemical staining revealed the appearance of Pax6⁺ neurogenic niches and a larger number of neural precursor cells, which are mainly from Pax6⁺ radial glia cells, in the nuclei of the diencephalon and optic tectum of rainbow trout (Oncorhynchus mykiss). Taken together, unilateral eye injury can cause optic nerve reaction, and the formation of neurogenic niches is likely a compensation phenomenon during the repair process of optic nerve injury in rainbow trout (Oncorhynchus mykiss).

Involvement of pax2 in ovarian development and recrudescence of catfish: a role in steroidogenesis

PAX2, a member of paired box family, is an essential transcription factor for the organ development in vertebrates including teleosts, yet no evidence has been shown for its involvement in reproduction. To study this, partial- and/or full-length cDNA of pax2 was isolated from the ovary of catfish, Clarias batrachus, along with its other Pax family members, pax1 and pax9 Tissue distribution and ontogeny expression analysis indicated the prevalence of pax2 but not pax1 and pax9 in ovary. Varied phase-wise expression during ovarian cycle and elevation of pax2 after human chorionic gonadotropin induction showed probable regulation by gonadotropins. Pax2 could be localized in various stages of oocytes and in follicular layer of vitellogenic and post-vitellogenic oocytes. To assess the functional significance of pax2, transient RNA silencing was performed using primary catfish ovarian follicle culture, in vitro, and in catfish, in vivo, through ovary-targeted injection of PEI-esiRNA. Pax2 siRNA treatment reduced the expression of various transcripts related to ovarian development like signaling molecules such as wnt4 and wnt5, estrogen receptors, several steroidogenic enzymes and transcription factors. These transitions in transcript levels might have been mediated by Pax2 acting upstream of wnt4/5 that may play a role in steroidogenesis and/or ovarian development along with ad4bp/sf-1 or by direct or indirect interaction with steroidogenic enzyme genes, which is evident from the change in the levels of serum estradiol-17β but not 17α,20β-dihydroxy-4-pregnen-3-one. Taken together, it seems that pax2 has a plausible role during ovarian development and/or recrudescence of catfish either directly or indirectly through Wnt signaling pathway.

Promotion on the differentiation of retinal Müller cells into retinal ganglion cells by Brn-3b

AIM

To investigate the role of Brn-3b in differentiation process of stem cells derived from retinal Müller cells into the ganglion cell.

METHODS

The passage culture method of Müller cells from retina of newborn Sprague Dawley rats was carried out by repeated incomplete pancreatic enzyme digestion method. The cells were detected by fluorescence-activated cell sorter (FACS), immunohistochemistry technology and reverse transcription-polymerase chain reaction (RT-PCR) to determine the purity. The third passage of cells was induced in the serum-free dedifferentiation medium. The expression of the specific markers Ki-67 and nestin of retinal stem cells was measured by RT-PCR and Western blot. The cell proliferation of retinal stem cells was detected by 5-ethynyl-2'-deoxyuridine (Edu) staining. The cells were randomly divided into 5 groups as follows: group A: Brn-3bsiRNA group; group B: Brn-3b control siRNA group; group C: pGC-Brn-3b-green fluorescent protein (GFP) group; group D: pGC-GFP group; group E: control group (without any handling). The purified Müller cells were cultured for 3-7d, then, the percentage of ganglion cells was counted by immunofluorescence staining.

RESULTS

FACS demonstrated the purity of retinal Müller cells was more 97.44%. A few spherical cell spheres appeared. Immunofluorescence staining showed that stem cells within the spheres were positive for retinal stem cell-specific markers nestin (red fluorescence, 92.94%±6.48%) and Ki-67 (green fluorescence, 85.96%±6.04%). Meanwhile, RT-PCR analysis showed cell spheres in the culture to have expressed a battery of transcripts characteristic of stem cells such as nestin and Ki-67, which were absent in the Müller cells. Western blot analysis further confirmed the expression of nestin and Ki-67 in the cell spheres but not in the Müller cells. Edu staining showed most of the nuclei within the cell spheres were stained red (82.80%±6.65%), suggesting the new cell spheres had the capacity for effective proliferation. The statistics result showed the difference between Brn-3bsiRNA group and Brn-3b control siRNA group or the control group was significant (F=15, P<0.05), while the difference between Brn-3b control siRNA group or the control group was not statistically significant (P>0.05).

CONCLUSION

The repeated incomplete pancreatic enzyme digestion method is an efficient and practical method to purify retinal Müller cells. Retinal stem cells were successfully cloned in the dedifferentiational medium. Retinal Müller cells are accessible sources of retinal stem cells. Brn-3b is an important regulatory gene in stem cells differentiated into retinal ganglion cell.

Sox10 Expression in Goldfish Retina and Optic Nerve Head in Controls and after the Application of Two Different Lesion Paradigms

The mammalian central nervous system (CNS) is unable to regenerate. In contrast, the CNS of fish, including the visual system, is able to regenerate after damage. Moreover, the fish visual system grows continuously throughout the life of the animal, and it is therefore an excellent model to analyze processes of myelination and re-myelination after an injury. Here we analyze Sox10⁺ oligodendrocytes in the goldfish retina and optic nerve in controls and after two kinds of injuries: cryolesion of the peripheral growing zone and crushing of the optic nerve. We also analyze changes in a major component of myelin, myelin basic protein (MBP), as a marker for myelinated axons. Our results show that Sox10⁺ oligodendrocytes are located in the retinal nerve fiber layer and along the whole length of the optic nerve. MBP was found to occupy a similar location, although its loose appearance in the retina differed from the highly organized MBP⁺ axon bundles in the optic nerve. After optic nerve crushing, the number of Sox10⁺ cells decreased in the crushed area and in the optic nerve head. Consistent with this, myelination was highly reduced in both areas. In contrast, after cryolesion we did not find changes in the Sox10⁺ population, although we did detect some MBP^- degenerating areas. We show that these modifications in Sox10⁺ oligodendrocytes are consistent with their role in oligodendrocyte identity, maintenance and survival, and we propose the optic nerve head as an excellent area for research aimed at better understanding of de- and remyelination processes.