Loss of Active Neurogenesis in the Adult Shark Retina

Localization and Characterization of Major Neurogenic Niches in the Brain of the Lesser-Spotted Dogfish Scyliorhinus canicula

Adult neurogenesis is defined as the ability of specialized cells in the postnatal brain to produce new functional neurons and to integrate them into the already-established neuronal network. This phenomenon is common in all vertebrates and has been found to be extremely relevant for numerous processes, such as long-term memory, learning, and anxiety responses, and it has been also found to be involved in neurodegenerative and psychiatric disorders. Adult neurogenesis has been studied extensively in many vertebrate models, from fish to human, and observed also in the more basal cartilaginous fish, such as the lesser-spotted dogfish, Scyliorhinus canicula, but a detailed description of neurogenic niches in this animal is, to date, limited to the telencephalic areas. With this article, we aim to extend the characterization of the neurogenic niches of S. canicula in other main areas of the brain: we analyzed via double immunofluorescence sections of telencephalon, optic tectum, and cerebellum with markers of proliferation (PCNA) and mitosis (pH3) in conjunction with glial cell (S100β) and stem cell (Msi1) markers, to identify the actively proliferating cells inside the neurogenic niches. We also labeled adult postmitotic neurons (NeuN) to exclude double labeling with actively proliferating cells (PCNA). Lastly, we observed the presence of the autofluorescent aging marker, lipofuscin, contained inside lysosomes in neurogenic areas.

Use of vivo-morpholinos for gene knockdown in the postnatal shark retina

Work in the catshark Scyliorhinus canicula has shown that the evolutionary origin of postnatal neurogenesis in vertebrates is earlier than previously thought. Thus, the catshark can serve as a model of interest to understand postnatal neurogenic processes and their evolution in vertebrates. One of the best characterized neurogenic niches of the catshark CNS is found in the peripheral region of the retina. Unfortunately, the lack of genetic tools in sharks limits the possibilities to deepen in the study of genes involved in the neurogenic process. Here, we report a method for gene knockdown in the juvenile catshark retina based on the use of Vivo-Morpholinos. To establish the method, we designed Vivo-Morpholinos against the proliferation marker PCNA. We first evaluated the possible toxicity of 3 different intraocular administration regimes. After this optimization step, we show that a single intraocular injection of the PCNA Vivo-Morpholino decreases the expression of PCNA in the peripheral retina, which leads to reduced mitotic activity in this region. This method will help in deciphering the role of other genes potentially involved in postnatal neurogenesis in this animal model.

Developmental genoarchitectonics as a key tool to interpret the mature anatomy of the chondrichthyan hypothalamus according to the prosomeric model

The hypothalamus is a key vertebrate brain region involved in survival and physiological functions. Understanding hypothalamic organization and evolution is important to deciphering many aspects of vertebrate biology. Recent comparative studies based on gene expression patterns have proposed the existence of hypothalamic histogenetic domains (paraventricular, TPa/PPa; subparaventricular, TSPa/PSPa; tuberal, Tu/RTu; perimamillary, PM/PRM; and mamillary, MM/RM), revealing conserved evolutionary trends. To shed light on the functional relevance of these histogenetic domains, this work aims to interpret the location of developed cell groups according to the prosomeric model in the hypothalamus of the catshark Scyliorhinus canicula, a representative of Chondrichthyans (the sister group of Osteichthyes, at the base of the gnathostome lineage). To this end, we review in detail the expression patterns of ScOtp, ScDlx2, and ScPitx2, as well as Pax6-immunoreactivity in embryos at stage 32, when the morphology of the adult catshark hypothalamus is already organized. We also propose homologies with mammals when possible. This study provides a comprehensive tool to better understand previous and novel data on hypothalamic development and evolution.

Embryonic nutritional hyperglycemia decreases cell proliferation in the zebrafish retina

Diabetic retinopathy (DR) is one of the leading causes of blindness in the world. While there is a major focus on the study of juvenile/adult DR, the effects of hyperglycemia during early retinal development are less well studied. Recent studies in embryonic zebrafish models of nutritional hyperglycemia (high-glucose exposure) have revealed that hyperglycemia leads to decreased cell numbers of mature retinal cell types, which has been related to a modest increase in apoptotic cell death and altered cell differentiation. However, how embryonic hyperglycemia impacts cell proliferation in developing retinas still remains unknown. Here, we exposed zebrafish embryos to 50 mM glucose from 10 h postfertilization (hpf) to 5 days postfertilization (dpf). First, we confirmed that hyperglycemia increases apoptotic death and decreases the rod and Müller glia population in the retina of 5-dpf zebrafish. Interestingly, the increase in cell death was mainly observed in the ciliary marginal zone (CMZ), where most of the proliferating cells are located. To analyze the impact of hyperglycemia in cell proliferation, mitotic activity was first quantified using pH3 immunolabeling, which revealed a significant decrease in mitotic cells in the retina (mainly in the CMZ) at 5 dpf. A significant decrease in cell proliferation in the outer nuclear and ganglion cell layers of the central retina in hyperglycemic animals was also detected using the proliferation marker PCNA. Overall, our results show that nutritional hyperglycemia decreases cellular proliferation in the developing retina, which could significantly contribute to the decline in the number of mature retinal cells.

Zebrafish: An In Vivo Screening Model to Study Ocular Phenotypes

Purpose

To establish a set of assays that allow the in vivo screening of candidate genes for ocular diseases in zebrafish, with an emphasis on refractive error.

Methods

Our pipeline includes the most relevant ocular screening measurements to assess (1) ocular biometry using spectral domain optical coherence tomography, (2) refractive status using an eccentric photorefractor, (3) intraocular pressure by tonometry, and (4) optokinetic response to study visual capability in zebrafish. To validate our pipeline and to demonstrate the potential of zebrafish as a valid animal model, we chose two well-characterized genes with an ocular phenotype (PRSS56 and FBN1) and generated two mutant zebrafish lines (prss56 and fbn1). Mutant fish were assessed at 2, 4, and 6 months after fertilization.

Results

With the proposed phenotyping pipeline, we showed that ocular biometry, refractive status, intraocular pressure, and visual function can be studied in zebrafish. In the prss56 mutant, the pipeline revealed a dramatic decrease in axial length, mainly owing to a decreased vitreous chamber depth, whereas in the fbn1 mutant, ectopia lentis was the most distinctive ocular phenotype observed. Tonometry in both mutant lines showed an increase in intraocular pressure.

Conclusions

The proposed pipeline was applied successfully in zebrafish and can be used for future genetic screenings of candidate genes. While validating our pipeline, we found a close resemblance between the ocular manifestations in the zebrafish mutants and patients harboring mutations in PRSS56 and FBN1. Our results support the validity of our pipeline and highlight the potential of zebrafish as an animal model for in vivo screening of candidate genes for ocular diseases.

Decline in Constitutive Proliferative Activity in the Zebrafish Retina with Ageing

It is largely assumed that the teleost retina shows continuous and active proliferative and neurogenic activity throughout life. However, when delving into the teleost literature, one finds that assumptions about a highly active and continuous proliferation in the adult retina are based on studies in which proliferation was not quantified in a comparative way at the different life stages or was mainly studied in juveniles/young adults. Here, we performed a systematic and comparative study of the constitutive proliferative activity of the retina from early developing (2 days post-fertilisation) to aged (up to 3–4 years post-fertilisation) zebrafish. The mitotic activity and cell cycle progression were analysed by using immunofluorescence against pH3 and PCNA, respectively. We observed a decline in the cell proliferation in the retina with ageing despite the occurrence of a wave of secondary proliferation during sexual maturation. During this wave of secondary proliferation, the distribution of proliferating and mitotic cells changes from the inner to the outer nuclear layer in the central retina. Importantly, in aged zebrafish, there is a virtual disappearance of mitotic activity. Our results showing a decline in the proliferative activity of the zebrafish retina with ageing are of crucial importance since it is generally assumed that the fish retina has continuous proliferative activity throughout life.

Retinal Stem Cell 'Retirement Plans': Growth, Regulation and Species Adaptations in the Retinal Ciliary Marginal Zone

The vertebrate retina develops from a specified group of precursor cells that adopt distinct identities and generate lineages of either the neural retina, retinal pigmented epithelium, or ciliary body. In some species, including teleost fish and amphibians, proliferative cells with stem-cell-like properties capable of continuously supplying new retinal cells post-embryonically have been characterized and extensively studied. This region, termed the ciliary or circumferential marginal zone (CMZ), possibly represents a conserved retinal stem cell niche. In this review, we highlight the research characterizing similar CMZ-like regions, or stem-like cells located at the peripheral margin, across multiple different species. We discuss the proliferative parameters, multipotency and growth mechanisms of these cells to understand how they behave in vivo and how different molecular factors and signalling networks converge at the CMZ niche to regulate their activity. The evidence suggests that the mature retina may have a conserved propensity for homeostatic growth and plasticity and that dysfunction in the regulation of CMZ activity may partially account for dystrophic eye growth diseases such as myopia and hyperopia. A better understanding of the properties of CMZ cells will enable important insight into how an endogenous generative tissue compartment can adapt to altered retinal physiology and potentially even restore vision loss caused by retinal degenerative conditions.