To shed light on the cellular and molecular mechanisms of zebrafish adult neurogenesis and regeneration, we developed a protocol for invasive surgery causing mechanical injuries in the zebrafish adult telencephalon and subsequent monitoring of changes in the stabbed hemisphere by immunohistochemistry or in situ hybridization.
Adult zebrafish have an amazing capacity to regenerate their central nervous system after injury. To investigate the cellular response and the molecular mechanisms involved in zebrafish adult central nervous system (CNS) regeneration and repair, we developed a zebrafish model of adult telencephalic injury.
In this approach, we manually generate an injury by pushing an insulin syringe needle into the zebrafish adult telencephalon. At different post injury days, fish are sacrificed, their brains are dissected out and stained by immunohistochemistry and/or in situ hybridization (ISH) with appropriate markers to observe cell proliferation, gliogenesis, and neurogenesis. The contralateral unlesioned hemisphere serves as an internal control. This method combined for example with RNA deep sequencing can help to screen for new genes with a role in zebrafish adult telencephalon neurogenesis, regeneration, and repair.
Mammals have a very limited capacity to generate new neurons during adult life, and adult neurogenesis in the brain is mostly restricted to two telencephalic regions situated at the subventricular zone (SVZ) of the lateral ventricles, and the subgranular zone (SGZ) of the dentate gyrus in the hippocampus1. Mammals also cannot repair damage of the brain caused by neurodegenerative diseases, stroke, or injuries, efficiently. Lost neurons are not replaced and instead proliferation of different types of glial cells, including astrocytes, microglia, and oligodendrocytes, is observed. As a consequence of this proliferative process called reactive gliosis, the injured brain tissue is sealed off by the formation of a glial scar, which inhibits neuronal and axonal regeneration2-4.
In contrast to mammals, teleost fish like the zebrafish form new neurons abundantly in adult stages and have a high regenerative and proliferative potential to repair lesions of the central nervous system2,5-7. The zebrafish adult brain contains 16 distinct neural stem cell niches that generate a large number of new neurons during the entire life8-11. Neurons born in these specific proliferation zones of the adult zebrafish brain migrate to their target areas, where they will mature and integrate into the existing neuronal networks8,10,12-15.
Among the progenitor zones identified in adult zebrafish, the telencephalic ventricular zone is one of the most studied neuronal stem cell niches. Cells in this progenitor niche can be classified into three distinct types regarding their morphology, division rate and expression of different glial or neuronal markers. Type I and type II cells are radial glial like cells which have long processes. They express stem cells markers including GFAP, nestin, and S100β. Type I cells do not proliferate while type II cells proliferate slowly, as evidenced by their expression of proliferation markers such as proliferating cell nuclear antigen (PCNA) and the incorporation of desoxythymidine analogues. The type III cells are fast dividing cells that are committed to become neural progenitors and express neural marker genes, such as PSA-NCAM5,16.
Although the regenerative capacities of teleost fish are well documented, only few publications have described and investigated in detail the cellular and molecular mechanisms involved in neuronal regeneration in the adult telencephalon in response to injury15,17-22. To decipher the mechanisms implicated in zebrafish adult central nervous system regeneration and repair and to understand the similarities and the differences between constitutive and regenerative neurogenesis, we developed a zebrafish model of adult telencephalic injury. Here, we will describe visually how to generate an injury in one of the telencephalic hemispheres with the help of a syringe needle, while keeping the contralateral unlesioned side as an internal control. We will also show how to monitor changes upon injury in cell proliferation and gene expression by immunohistochemistry and in situ hybridization assays.
Zebrafish were maintained in the fish facility of the Institute of Toxicology and Genetics (ITG) at Karlsruhe Institute of Technology (KIT). Experiments on animals were performed in accordance with the German animal protection standards and were approved by the Government of Baden-Württemberg, Regierungspräsidium Karlsruhe, Germany (Aktenzeichen 35-9185.81/G-272/12 'Adulte Neurogenese').
1. Generating a Stab Wound in the Telencephalon
2. Analyzing the Effect of Telencephalic Injury
Using the workflow described in this article, a lesion was introduced into the right hemisphere of an adult zebrafish telencephalon (Figures 1A-1B). A correct wounding leads to a lesion canal that extends from dorsal to ventral through the whole pallium of the telencephalon (Figure 3C). The lesion should be restricted to one of the hemispheres and should not cross the medial ventricle. The lesion canal is visible with bright field microscopy (Figure 3C) and will be healed after approximately 35 days15.
It was previously shown that after wounding an up-regulation of proliferating cell nuclear antigen (PCNA; Figure 3A) and S100β (Figure 3B) at 3 to 7 dpl is detectable by immunohistochemistry15. The staining of PCNA and S100β is overlapping, indicating proliferation of radial glial cells.
Furthermore, in order to visualize oligodendrocyte precursor cells (OPCs), a stab wound assay was performed on the Tg(olig2:EGFP) fish24. Upon staining with an anti-GFP antibody, accumulation of OPCs in close proximity to the lesion canal is detectable (Figure 3D). This accumulation is transient, and OPC clusters are not observed anymore at 35 dpl15.
If the lesion was not introduced properly, the lesion canal will not be visible, and up-regulation of PCNA and S100β or accumulation of OPCs will not be detectable. PCNA is the most sensitive marker for brain injury. In order to verify the efficiency of the stab wound and to exclude that both hemispheres are wounded, it is strongly recommended to always stain the sections with an anti-PCNA antibody. In a correctly introduced lesion, PCNA should be significantly up-regulated (up to 4-fold15) in only one of the hemispheres. Although in most cases it is sufficient to only use the contralateral unlesioned side as an internal control to monitor changes in gene expression, it is highly recommended to compare the gene expression in the control hemisphere with the expression in telencephalic hemispheres of wild type brains. In this way, any systemic effects of the lesion on gene expression that would affect both hemispheres can be detected.
The stab wound assay in combination with a systematic expression screen of transcription regulators (TRs) (25 and Schmidt et al., unpublished) can also be used to identify TR genes with a possible role in neurogenesis and regeneration (Figures 4A-4C).
This approach has identified a number of factors that are expressed in the telencephalon and whose expression is specifically up-regulated in response to injury (Figure 4A-C).
Figure 1. Schematic representation of adult zebrafish telencephalic stab wounding. A) An insulin syringe needle is pushed through the skull of an adult zebrafish to generate a telencephalic injury. The white dashed line represents the border between the two telencephalic hemispheres and the green cross indicates the position of the stab wound. B) Dorsal view of an adult zebrafish brain. The major brain areas are indicated. The green cross shows the position of the induced injury. C) The tip of the needle, which serves to induce the telencephalic injury, measures 2 mm. Olfactory bulb (ob), telencephalon (tel) and optic tectum (ot).
Figure 2. Schematic representation of an adult zebrafish brain in a trimmed agarose block just before vibratome sectioning. Anterior is up. Olfactory bulb (ob), telencephalon (tel), optic tectum (ot), cerebellum (cb) and medulla (m). Scale bar: 500 μm.
Figure 3. Regenerative responses to stab wound injury. A-D) Adult zebrafish brain at 5 days post lesion (dpl). A-B) The proliferative marker PCNA (A) and the radial glial marker S100β (B) are up-regulated at the ventricular zone (arrows) upon stab wounding. (C) The lesion channel is visible under bright field illumination (arrows). The ventricular zone is indicated by the green line. (D) Oligodendrocyte precursor cells (olig2:EGFP+) accumulate at the site of lesion (arrows). Scale bar: 200 μm. In all panels, the right hemisphere is the lesioned hemisphere. Dorsal is up. The dashed line in A to D indicates the border between the pallium and the subpallium.
Figure 4. An example of genes up-regulated in response to stab injury. Note the strong up-regulation (arrows) of eomesa (A, B) and sox4a (C) mRNA expression in the right telencephalic hemisphere (stabbed hemisphere) as monitored by in situ hybridization at 5 dpl. The left hemisphere is uninjured and serves as control. (B) Shows a higher magnification of the dorsal telencephalic region of A. Dorsal is up. Scale bar: 200 μm for A and C and 55 µm for B. The dashed line in A and C indicates the separation between the pallium and the subpallium. Please click here to view a larger version of this figure.
ABBREVIATIONS | |
AP | Alkaline Phosphatase |
BCIP | 5-bromo-4-chloro-3'-indolyphosphate |
BSA | Bovine serum albumin |
CNS | Central nervous system |
Dig | Digoxygenin |
DMSO | Dimethyl sulfoxide |
dpl | Days post lesion |
eomesa | Eomesodermin homolog a |
GFAP | Glial fibrillary acidic protein |
hr | Hour |
HYB buffer | Hybridization buffer |
ISH | in situ hybridization |
MeOH | Methanol |
NBT | Nitro-blue tetrazolium |
OPCs | Oligodendrocyte precursor cells |
PBS | Phosphate buffered saline |
PCNA | Proliferating cell nuclear antigen |
PFA | Paraformaldehyde |
ProtK | Proteinase K |
PSA-NCAM | Polysialylated neuronal cell adhesion molecule |
PTW | PBS, 0.1% Tween-20 |
RNA | Ribonucleic acid |
RT | Room temperature |
SGZ | Subgranular zone |
sox4a | SRY-box containing gene 4a |
SSC | Saline-sodium citrate |
SVZ | Subventricular zone |
TRs | Transcription regulators |
We describe here a method to induce a telencephalic injury by pushing a needle through the skull of the adult zebrafish, and to analyze the cellular and molecular reaction to this injury by immunohistochemistry and in situ hybridization. The main advantage of this technique over other existing approaches is its simplicity, speed and cost efficiency. Therefore, this technique, which allows production of many injured brains in a short time, can be easily combined with a large scale in situ hybridization screen to identify genes with a role in regeneration and neurogenesis.
The age of the adult zebrafish on which the injury is performed is crucial. If the fish are older than 1.5 years, the skull is too hard and it is possible that the skull is being pushed down or fractured rather than cleanly perforated. In contrast, in younger fish (younger than 4 months), there is still a lot of neurogenesis going on, which might lead to false positive results regarding reactive neurogenesis.
Another critical step in this approach is to verify the efficiency of the stab wound and to exclude that both hemispheres are wounded. This can be monitored by staining the sections with the anti-PCNA antibody. In a correctly introduced lesion, PCNA should be significantly up-regulated in only one of the hemispheres.
It is important to bear in mind that the induced mechanical injury will have several side effects such as non-specific cell ablation, increased cell death because of secondary degeneration, inflammation and destruction of the blood-brain barrier, and damage of ventricular zone and as a possible consequence flow of the cerebrospinal fluid into the brain parenchyma. However, the injured fish appear to suffer very little from the injury. Within 2 min they are free swimming again and normal feeding behavior is restored within about 4 hr. After 3 weeks the injury is morphologically not visible anymore. In our hands, from several hundred surgeries we did not lose a single fish despite the fact that we had a 100% injury rate as assessed by histology at 5 days post-lesion.
In the last years other techniques such as transgenic approaches have been developed in zebrafish to produce tissue or cell type specific ablation2. Although these techniques are very elegant and minimally invasive they are based on the generation of complicated DNA constructs and the establishment of several stable zebrafish transgenic lines which can be very tedious and time consuming. Therefore, simple mechanical approaches such as the assay described in this protocol remain an important tool to study adult brain neurogenesis and regeneration.
The authors have nothing to disclose.
We thank N. Borel and her fish house team, M. Rastegar for help with microscopy, and T. Dickmeis for comments on the manuscript. We are grateful for support by the EU IP ZF-Health, FP7-HEALTH-2007-B2, NeuroXsys, the Interreg Network for Synthetic Biology in the Upper Rhine valley (NSB-Upper Rhine) and the BMBF funded network EraSysBio.
Name of Material/ Equipment | Company | Catalog Number | Comments/Description |
Tricaine | Sigma | 335.2 | |
10x DPBS (-/-) | Life technologies | 14200-083 | |
30 G syringe (Omnican 40) | B.Braun Melsungen | 916162 | |
Polyethylene molding cup tray | Polysciences, Inc | 17177C-3 | |
PeqGOLD Universal Agarose | Peqlab | 35-1020 | |
Bovine serum albumin | Biochrom | W 2835919108 | |
Aqua Poly/Mount | Polysciences, Inc | 18606-20 | |
Tween 20 | Carl Roth | 9127.1 | |
DMSO | Carl Roth | A994.2 | |
Dissection needle | Carl Roth | KX93.1 | |
Paraformaldehyde | Carl Roth | 335.2 | |
Loctite 414 | Schöffler und Wörner | 06 1362 0020 | |
Glass slides | Labonord | 5305103 | |
Cover slips 24 x 60 mm | Labonord | 5305060 | |
Petri dishes 94 mm | Greiner bio-one | 632180 | |
Falcons 15 ml | Greiner bio-one | 188261 | |
24-well plate | Greiner bio-one | 662160 | |
Anti-S100 (1:500) | Dako | Z031101-2 | |
Anti-PCNA (1:400) | Dako | M087901 | |
Anti-GFP (1 :1000) | Aves Labs | GFP-1020 | |
anti-Dig AP-coupled antibody | Roche Diagnostics | 11093274910 | |
5-Bromo-4-chloro-3-indolyl-phosphate (BCIP) | Roche Diagnostics | 1383221 | |
4-Nitroblue tetrazolium chloride (NBT) | Roche Diagnostics | 1383213 | |
Proteinase K | Roche Diagnostics | 1092766 | |
Polyethylene molding cup tray | Polysciences | 17177C-3 | |
Vibratome VT1000S | Leica | ||
Confocal microscope Sp5 DM6000 | Leica | ||
Dissecting microscope Nikon SMZ 645 | Nikon | ||
24-well flat bottomed tissue culture plates | Falcon catalog | 353226 | |
Student Dumont #5 Forceps | Fine Science Tools | 91150-20 | |
Surgical scissors | Fine Science Tools | 91460-11 | |
Danio rerio Tg(olig2:EGFP) | 24 | ||
Danio rerio WT strains (e.g. AB2O2) |