Ex vivo electroretinography (ERG) provides insight into the health and functionality of retinal cells, the integrity of phototransduction, and the visual cycle and allows for the direct application of pharmaceuticals to the retinal tissues. Here, we present a protocol for performing ex vivo ERG on adult zebrafish. We describe steps for zebrafish tissue dissection, mounting tissues, and assembling and connecting cassettes. We then detail procedures for running the Diagnosys software and processing and analyzing the resulting data.

Cellular retinaldehyde binding protein (CRALBP) plays a crucial role in the visual cycle by chaperoning 11-cis-retinoids. Mutations in its encoding gene RLBP1 lead to inherited retinal diseases with the common feature of poor night vision. Zebrafish possess two RLBP1 paralogs, rlbp1a and rlbp1b, with distinct retinal expression profiles, providing a bespoke opportunity to dissect cell-specific functions of CRALBP. Here, we first resolved conflicting reports on paralog expression by interrogating zebrafish single-cell RNA-sequencing datasets, which revealed predominant rlbp1a expression in Müller glia and rlbp1b expression in the RPE. Using CRISPR-generated zebrafish knockouts, we demonstrated that loss of RPE-expressed rlbp1b selectively impaired optokinetic responses (OKR) with a ~50% reduction in saccade frequency relative to wildtype. This impaired OKR response is only seen when under dim light conditions with no defect seen in standard or bright light rearing conditions. This recapitulates the night blindness presentation in patients with RLBP1 mutations. Retinoid profiling of rlbp1b knockout larvae showed significant decreases in 11-cis-retinal (62% reduced) and all-trans-retinal (69% reduced) levels. To explore mechanistic changes following rlbp1b loss, unbiased proteomic profiling was carried out on rlbp1b knockout adult zebrafish eyes. This confirmed the knockout of Cralbpb and revealed significant disruption of proteins involved in vitamin A metabolism, lipid metabolism/storage and ferroptosis. To explore the utility of zebrafish for in vivo pathogenicity assessment of RLBP1, we established a complementation assay using transgenic zebrafish. Although expression of wildtype EGFP-tagged human RLBP1 did not rescue the visual deficit, expression of zebrafish Cralbp to the RPE restored dim light vision, whereas zebrafish Cralbp harboring the human pathogenic p.R151Q mutation failed to do so. Together, these findings underscore the predominant role of RPE-expressed CRALBP in sustaining visual function under low-light conditions and establish a zebrafish platform for functional evaluation of RLBP1 variants.

Cellular retinaldehyde binding protein (CRALBP) plays a crucial role in the visual cycle by chaperoning 11-cis-retinoids. Mutations in its encoding gene RLBP1 lead to inherited retinal diseases with the common feature of poor night vision. Zebrafish possess two RLBP1 paralogs, rlbp1a and rlbp1b, with distinct retinal expression profiles, providing a bespoke opportunity to dissect cell-specific functions of CRALBP. Here, we first resolved conflicting reports on paralog expression by interrogating zebrafish single-cell RNA-sequencing datasets, which revealed predominant rlbp1a expression in Müller glia and rlbp1b expression in the RPE. Using CRISPR-generated zebrafish knockouts, we demonstrated that loss of RPE-expressed rlbp1b selectively impaired optokinetic responses (OKR) with a ~50% reduction in saccade frequency relative to wildtype. This impaired OKR response is only seen when under dim light conditions with no defect seen in standard or bright light rearing conditions. This recapitulates the night blindness presentation in patients with RLBP1 mutations. Retinoid profiling of rlbp1b knockout larvae showed significant decreases in 11-cis-retinal (62% reduced) and all-trans-retinal (69% reduced) levels. To explore mechanistic changes following rlbp1b loss, unbiased proteomic profiling was carried out on rlbp1b knockout adult zebrafish eyes. This confirmed the knockout of Cralbpb and revealed significant disruption of proteins involved in vitamin A metabolism, lipid metabolism/storage and ferroptosis. To explore the utility of zebrafish for in vivo pathogenicity assessment of RLBP1, we established a complementation assay using transgenic zebrafish. Although expression of wildtype EGFP-tagged human RLBP1 did not rescue the visual deficit, expression of zebrafish Cralbp to the RPE restored dim light vision, whereas zebrafish Cralbp harboring the human pathogenic p.R151Q mutation failed to do so. Together, these findings underscore the predominant role of RPE-expressed CRALBP in sustaining visual function under low-light conditions and establish a zebrafish platform for functional evaluation of RLBP1 variants.

Mutations in CLRN1 cause Usher syndrome type IIIA (USH3A), an autosomal recessive disorder characterized by hearing and vision loss, and often accompanied by vestibular dysfunction. The identity of the cell types responsible for the pathology and mechanisms leading to vision loss in USH3A remains elusive. To address this, we employed CRISPR/Cas9 technology to delete a large region in the coding and untranslated (UTR) region of zebrafish clrn1. The retinas of clrn1 mutant larvae exhibited sensitivity to cell stress, along with age-dependent loss of function and degeneration in the photoreceptor layer. Investigation revealed disorganization in the outer retina in clrn1 mutants, including actin-based structures of the Müller glia and photoreceptor cells. To assess cell-specific contributions to USH3A pathology, we specifically re-expressed clrn1 in either Müller glia or photoreceptor cells. Müller glia re-expression of clrn1 prevented the elevated cell death observed in larval clrn1 mutant zebrafish exposed to high-intensity light. Notably, the degree of phenotypic rescue correlated with the level of Clrn1 re-expression. Surprisingly, high levels of Clrn1 expression enhanced cell death in both wild-type and clrn1 mutant animals. However, rod- or cone-specific Clrn1 re-expression did not reduce the extent of cell death. Taken together, our findings underscore three crucial insights. First, clrn1 mutant zebrafish exhibit key pathological features of USH3A; second, Clrn1 within Müller glia plays a pivotal role in photoreceptor maintenance, with its expression requiring controlled regulation; third, the reliance of photoreceptors on Müller glia suggests a structural support mechanism, possibly through direct interactions between Müller glia and photoreceptors mediated in part by Clrn1 protein.

Inherited retinal diseases (IRDs) are a rare group of eye disorders characterized by progressive dysfunction and degeneration of retinal cells. In this study, we characterized the raifteirí (raf) zebrafish, a novel model of inherited blindness, identified through an unbiased ENU mutagenesis screen. A mutation in the largest subunit of the endoplasmic reticulum membrane protein complex, emc1 was subsequently identified as the causative raf mutation. We sought to elucidate the cellular and molecular phenotypes in the emc1-/- knockout model and explore the association of emc1 with retinal degeneration. Visual behavior and retinal electrophysiology assays demonstrated that emc1-/- mutants had severe visual impairments. Retinal histology and morphometric analysis revealed extensive abnormalities, including thinning of the photoreceptor layer, in addition to large gaps surrounding the lens. Notably, photoreceptor outer segments were drastically smaller, outer segment protein expression was altered and hyaloid vasculature development was disrupted. Transcriptomic profiling identified cone and rod-specific phototransduction genes significantly downregulated by loss of emc1. These data shed light on why emc1 is a causative gene in inherited retinal disease and how outer segment morphogenesis is regulated.

Background:The goal of this study was to identify and characterize cell-cell interactions that facilitate endothelial tip cell fusion downstream of BMP (bone morphogenic protein)-mediated venous plexus formation. Methods:High resolution and time-lapse imaging of transgenic reporter lines and loss-of-function studies were carried out to study the involvement of mesenchymal stromal cells during venous angiogenesis. Results:BMP-responsive stromal cells facilitate timely and precise fusion of venous tip cells during developmental angiogenesis. Conclusions:Stromal cells are required for anastomosis of venous tip cells in the embryonic caudal hematopoietic tissue.

Outer segment phagocytosis (OSP) is a highly-regulated, biological process wherein photoreceptor outer segment (OS) tips are cyclically phagocytosed by the adjacent retinal pigment epithelium (RPE) cells. Often an overlooked retinal process, rhythmic OSP ensures the maintenance of healthy photoreceptors and vision. Daily, the photoreceptors renew OS at their base and the most distal, and likely oldest, OS tips, are phagocytosed by the RPE, preventing the accumulation of photo-oxidative compounds by breaking down phagocytosed OS tips and recycling useful components to the photoreceptors. Light changes often coincide with an escalation of OSP and within hours the phagosomes formed in each RPE cell are resolved. In the last two decades, individual molecular regulators were elucidated. Some of the molecular machinery used by RPE cells for OSP is highly similar to mechanisms used by other phagocytic cells for the clearance of apoptotic cells. Consequently, in the RPE, many molecular regulators of retinal phagocytosis have been elucidated. However, there is still a knowledge gap regarding the key regulators of physiological OSP in vivo between endogenous photoreceptors and the RPE. Understanding the regulation of OSP is of significant clinical interest as age-related macular degeneration (AMD) and inherited retinal diseases (IRD) are linked with altered OSP. Here, we review the in vivo timing of OSP peaks in selected species and focus on the reported in vivo environmental and molecular regulators of OSP.

Children that undergo intraocular surgery have an exaggerated postoperative response compared to adults that can result in significant postoperative challenges and reduced post-operative visual acuity. Rabbits were used as an animal model for investigating aging differences, treatment options, and surgical techniques for anterior chamber surgical interventions due to similarities in anterior chamber size and decreasing postoperative response with age. In our study, juvenile and adult rabbits underwent lensectomy with intraocular lens (IOL) insertion to determine how ocular RNA transcripts and proteins change with age. Rabbits underwent lensectomy with IOL insertion, and aqueous humor (AH) was collected immediately prior to surgery and at the peak of the postoperative response on post-operative day 3. Proteins related to coagulation and inflammation were assessed using targeted mass spectrometry. In addition, the cornea and iris/ciliary body tissues were dissected, and transcripts analyzed using RNA sequencing. While clinically, juvenile rabbits have greater fibrin formation following intraocular surgery compared to older rabbits, this change does not appear to be related to relative abundance levels of coagulation and inflammatory proteins in the AH. Gene transcript levels from a variety of immune response and inflammatory pathways reflected significant increases when comparing operated to unoperated ocular tissues, indicating the significant impact that surgery has on each ocular structure. This work further advances our understanding of how the rabbit eye proteomic and transcriptomic changes in response to surgery with aging, as we seek to ultimately identify the mechanisms for the exaggerated postoperative responses after pediatric intraocular surgery.

Purpose To assess the performance of two spectral-domain optical coherence tomography-angiography systems in a natural model of hypoperfusion: the hibernating thirteen-lined ground squirrel (13-LGS). Methods Using a high-speed (130 kHz) OCT-A system (HS-OCT-A) and a commercial OCT (36 kHz; Bioptigen Envisu; BE-OCT-A), we imaged the 13-LGS retina throughout its hibernation cycle. Custom software was used to extract the superior, middle, and deep capillary plexus (SCP, MCP, and DCP, respectively). The retinal vasculature was also imaged with adaptive optics scanning light ophthalmoscopy (AOSLO) during torpor to visualize individual blood cells. Finally, correlative histology with immunolabeled or DiI-stained vasculature was performed. Results During euthermia, vessel density was similar between devices for the SCP and MCP (P = 0.88, 0.72, respectively), with a small difference in the DCP (−1.63 ± 1.54%, P = 0.036). Apparent capillary dropout was observed during torpor, but recovered after forced arousal, and this effect was exaggerated in high-speed OCT-A imaging. Based on cell flux measurements with AOSLO, increasing OCT-A scan duration by ∼1000× would avoid the apparent capillary dropout artifact. High correspondence between OCT-A (during euthermia) and histology enabled lateral scale calibration. Conclusions While the HS-OCT-A system provides a more efficient workflow, the shorter interscan interval may render it more susceptible to the apparent capillary dropout artifact. Disambiguation between capillary dropout and transient ischemia can have important implications in the management of retinal disease and warrants additional diagnostics. Translational Relevance The 13-LGS provides a natural model of hypoperfusion that may prove valuable in modeling the utility of OCT-A in human pathologies associated with altered blood flow.

PURPOSE. Proper refractive development of the eye, termed emmetropization, is critical for focused vision and is impacted by both genetic determinants and several visual environment factors. Improper emmetropization caused by genetic variants can lead to congenital hyperopia, which is characterized by small eyes and relatively short ocular axial length. To date, variants in only four genes have been firmly associated with human hyperopia, one of which is MFRP. Zebrafish mfrp mutants also have hyperopia and, similar to reports in mice, exhibit increased macrophage recruitment to the retina. The goal of this research was to examine the effects of macrophage ablation on emmetropization and mfrp-related hyperopia. METHODS. We utilized a chemically inducible, cell-specific ablation system to deplete macrophages in both wild-type and mfrp mutant zebrafish. Spectral-domain optical coherence tomography was then used to measure components of the eye and determine relative refractive state. Histology, immunohistochemistry, and transmission electron microscopy were used to further study the eyes. RESULTS. Although macrophage ablation does not cause significant changes to the relative refractive state of wild-type zebrafish, macrophage ablation in mfrp mutants significantly exacerbates their hyperopic phenotype, resulting in a relative refractive error 1.3 times higher than that of non-ablated mfrp siblings. CONCLUSIONS. Genetic inactivation of mfrp leads to hyperopia, as well as abnormal accumulation of macrophages in the retina. Ablation of the mpeg1-positive macrophage population exacerbates the hyperopia, suggesting that macrophages may be recruited in an effort help preserve emmetropization and ameliorate hyperopia.

Interkinetic nuclear migration (IKNM) is the process in which pseudostratified epithelial nuclei oscillate from the apical to basal surface and in phase with the mitotic cycle. In the zebrafish retina, neuroepithelial retinal progenitor cells (RPCs) increase Notch activity with apical movement of the nuclei, and the depth of nuclear migration correlates with the probability that the next cell division will be neurogenic. This study focuses on the mechanisms underlying the relationships between IKNM, cell signaling, and neurogenesis. In particular, we have explored the role IKNM has on endosome biology within RPCs. Through genetic manipulation and live imaging in zebrafish, we find that early (Rab5-positive) and recycling (Rab11a-positive) endosomes polarize in a dynamic fashion within RPCs and with reference to nuclear position. Functional analyses suggest that dynamic polarization of recycling endosomes and their activity within the neuroepithelia modulates the subcellular localization of Crb2a, consequently affecting multiple signaling pathways that impact neurogenesis including Notch, Hippo, and Wnt activities. As nuclear migration is heterogenous and asynchronous among RPCs, Rab11a-affected signaling within the neuroepithelia is modulated in a differential manner, providing mechanistic insight to the correlation of IKNM and selection of RPCs to undergo neurogenesis.

Purpose To observe and characterize cone degeneration and regeneration in a selective metronidazole-mediated ablation model of ultraviolet-sensitive (UV) cones in zebrafish using in vivo optical coherence tomography (OCT) imaging. Methods Twenty-six sws1:nfsB-mCherry;sws2:eGFP zebrafish were imaged with OCT, treated with metronidazole to selectively kill UV cones, and imaged at 1, 3, 7, 14, 28, or 56 days after ablation. Regions 200 × 200 µm were cropped from volume OCT scans to count individual UV cones before and after ablation. Fish eyes were fixed, and immunofluorescence staining was used to corroborate cone density measured from OCT and to track monocyte response. Results Histology shows significant loss of UV cones after metronidazole treatment with a slight increase in observable blue cone density one day after treatment (Kruskal, Wallis, P = 0.0061) and no significant change in blue cones at all other timepoints. Regenerated UV cones measured from OCT show significantly lower density than pre-cone-ablation at 14, 28, and 56 days after ablation (analysis of variance, P < 0.01, P < 0.0001, P < 0.0001, respectively, 15.9% of expected nonablated levels). Histology shows significant changes to monocyte morphology (mixed-effects analysis, P < 0.0001) and retinal position (mixed-effects analysis, P < 0.0001). Conclusions OCT can be used to observe loss of individual cones selectively ablated by metronidazole prodrug activation and to quantify UV cone loss and regeneration in zebrafish. OCT images also show transient changes to the blue cone mosaic and inner retinal layers that occur concomitantly with selective UV cone ablation. Translational Relevance Profiling cone degeneration and regeneration using in vivo imaging enables experiments that may lead to a better understanding of cone regeneration in vertebrates.

The Forkhead Box C1 (FOXC1) gene encodes a forkhead/winged helix transcription factor involved in embryonic development. Mutations in this gene cause dysgenesis of the anterior segment of the eye, most commonly Axenfeld-Rieger syndrome (ARS), often with other systemic features. The developmental mechanisms and pathways regulated by FOXC1 remain largely unknown. There are two conserved orthologs of FOXC1 in zebrafish, foxc1a and foxc1b. To further examine the role of FOXC1 in vertebrates, we generated foxc1a and foxc1b single knockout zebrafish lines and bred them to obtain various allelic combinations. Three genotypes demonstrated visible phenotypes: foxc1a(-/-) single homozygous and foxc1(-/-) double knockout homozygous embryos presented with similar characteristics comprised of severe global vascular defects and early lethality, as well as microphthalmia, periocular edema and absence of the anterior chamber of the eye; additionally, fish with heterozygous loss of foxc1a combined with homozygosity for foxc1b (foxc1a(+/-);foxc1b(-)(/-)) demonstrated craniofacial defects, heart anomalies and scoliosis. All other single and combined genotypes appeared normal. Analysis of foxc1 expression detected a significant increase in foxc1a levels in homozygous and heterozygous mutant eyes, suggesting a mechanism for foxc1a upregulation when its function is compromised; interestingly, the expression of another ARS-associated gene, pitx2, was responsive to the estimated level of wild-type Foxc1a, indicating a possible role for this protein in the regulation of pitx2 expression. Altogether, our results support a conserved role for foxc1 in the formation of many organs, consistent with the features observed in human patients, and highlight the importance of correct FOXC1/foxc1 dosage for vertebrate development.