Stochastic single cell migration leads to robust horizontal cell layer formation in the vertebrate retina

Cell behaviors that pattern developing tissues: the case of the vertebrate nervous system

Morphogenesis from cells to tissue gives rise to the complex architectures that make our organs. How cells and their dynamic behavior are translated into functional spatial patterns is only starting to be understood. Recent advances in quantitative imaging revealed that, although highly heterogeneous, cellular behaviors make reproducible tissue patterns. Emerging evidence suggests that mechanisms of cellular coordination, intrinsic variability and plasticity are critical for robust pattern formation. While pattern development shows a high level of fidelity, tissue organization has undergone drastic changes throughout the course of evolution. In addition, alterations in cell behavior, if unregulated, can cause developmental malformations that disrupt function. Therefore, comparative studies of different species and of disease models offer a powerful approach for understanding how novel spatial configurations arise from variations in cell behavior and the fundamentals of successful pattern formation. In this chapter, I dive into the development of the vertebrate nervous system to explore efforts to dissect pattern formation beyond molecules, the emerging core principles and open questions.

Retinal neurons establish mosaic patterning by excluding homotypic somata from their dendritic territory

In vertebrate retina, individual neurons of the same type are distributed regularly across the tissue in a pattern known as a mosaic. Establishment of mosaics during development requires cell-cell repulsion among homotypic neurons, but the mechanisms underlying this repulsion remain unknown. Here we show that two mouse retinal cell types, OFF and ON starburst amacrine cells, establish mosaic spacing by using their dendritic arbors to repel neighboring homotypic somata. Using newly-generated transgenic tools and single cell labeling, we identify a transient developmental period when starburst somata receive extensive contacts from neighboring starburst dendrites; these serve to exclude somata from settling within the neighbor's dendritic territory. Dendrite-soma exclusion is mediated by MEGF10, a cell-surface molecule required for starburst mosaic patterning. Our results implicate dendrite-soma exclusion as a key mechanism underlying starburst mosaic spacing, and suggest that this could be a general mechanism for mosaic patterning across many cell types and species.

Neuronal migration prevents spatial competition in retinal morphogenesis

The concomitant occurrence of tissue growth and organization is a hallmark of organismal development1-3. This often means that proliferating and differentiating cells are found at the same time in a continuously changing tissue environment. How cells adapt to architectural changes to prevent spatial interference remains unclear. Here, to understand how cell movements that are key for growth and organization are orchestrated, we study the emergence of photoreceptor neurons that occur during the peak of retinal growth, using zebrafish, human tissue and human organoids. Quantitative imaging reveals that successful retinal morphogenesis depends on the active bidirectional translocation of photoreceptors, leading to a transient transfer of the entire cell population away from the apical proliferative zone. This pattern of migration is driven by cytoskeletal machineries that differ depending on the direction: microtubules are exclusively required for basal translocation, whereas actomyosin is involved in apical movement. Blocking the basal translocation of photoreceptors induces apical congestion, which hampers the apical divisions of progenitor cells and leads to secondary defects in lamination. Thus, photoreceptor migration is crucial to prevent competition for space, and to allow concurrent tissue growth and lamination. This shows that neuronal migration, in addition to its canonical role in cell positioning4, can be involved in coordinating morphogenesis.

Deterministic and probabilistic fate decisions co-exist in a single retinal lineage

Correct nervous system development depends on the timely differentiation of progenitor cells into neurons. While the output of progenitor differentiation is well investigated at the population and clonal level, how stereotypic or variable fate decisions are during development is still more elusive. To fill this gap, we here follow the fate outcome of single neurogenic progenitors in the zebrafish retina over time using live imaging. We find that neurogenic progenitor divisions produce two daughter cells, one of deterministic and one of probabilistic fate. Interference with the deterministic branch of the lineage affects lineage progression. In contrast, interference with fate probabilities of the probabilistic branch results in a broader range of fate possibilities than in wild-type and involves the production of any neuronal cell type even at non-canonical developmental stages. Combining the interference data with stochastic modelling of fate probabilities revealed that a simple gene regulatory network is able to predict the observed fate decision probabilities during wild-type development. These findings unveil unexpected lineage flexibility that could ensure robust development of the retina and other tissues.

Cell Fate of Retinal Progenitor Cells: In Ovo UbC-StarTrack Analysis

Clonal cell analysis outlines the ontogenic potential of single progenitor cells, allowing the elucidation of the neural heterogeneity among different cell types and their lineages. In this work, we analyze the potency of retinal stem/progenitor cells through development using the chick embryo as a model. We implemented in ovo the clonal genetic tracing strategy UbC-StarTrack for tracking retinal cell lineages derived from individual progenitors of the ciliary margin at E3.5 (HH21-22). The clonal assignment of the derived-cell progeny was performed in the neural retina at E11.5-12 (HH38) through the identification of sibling cells as cells expressing the same combination of fluorophores. Moreover, cell types were assessed based on their cellular morphology and laminar location. Ciliary margin derived-cell progenies are organized in columnar associations distributed along the peripheral retina with a limited tangential dispersion. The analysis revealed that, at the early stages of development, this region harbors multipotent and committed progenitor cells.

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.

Amoeboid-like migration ensures correct horizontal cell layer formation in the developing vertebrate retina

Migration of cells in the developing brain is integral for the establishment of neural circuits and function of the central nervous system. While migration modes during which neurons employ predetermined directional guidance of either preexisting neuronal processes or underlying cells have been well explored, less is known about how cells featuring multipolar morphology migrate in the dense environment of the developing brain. To address this, we here investigated multipolar migration of horizontal cells in the zebrafish retina. We found that these cells feature several hallmarks of amoeboid-like migration that enable them to tailor their movements to the spatial constraints of the crowded retina. These hallmarks include cell and nuclear shape changes, as well as persistent rearward polarization of stable F-actin. Interference with the organization of the developing retina by changing nuclear properties or overall tissue architecture hampers efficient horizontal cell migration and layer formation showing that cell-tissue interplay is crucial for this process. In view of the high proportion of multipolar migration phenomena observed in brain development, the here uncovered amoeboid-like migration mode might be conserved in other areas of the developing nervous system.

Cell migration orchestrates migrasome formation by shaping retraction fibers

Migrasomes are recently discovered vesicle-like structures on retraction fibers of migrating cells that have been linked with transfer of cellular contents, shedding of unwanted materials, and information integration. However, whether and how the cell migration paradigm regulates migrasome formation is not clear. Here, we report that there are significantly fewer migrasomes in turning cells compared with straight persistently migrating cells. The major insight underlying this observation is that as the cells elongate, their rear ends become narrower, subsequently resulting in fewer retraction fibers during impersistent migration. In addition to migration persistence, we reveal that migration speed positively corelates with migrasome formation, owing to the derived length of retraction fibers. Substantiating our hypothesis, genetically removing vimentin compromises cell migration speed and persistence and leads to fewer migrasomes. Together, our data explicate the critical roles of two cell migration patterns, persistence and speed, in the control of migrasome formation by regulating retraction fibers.

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.

Development and maintenance of vision's first synapse

Synapses in the outer retina are the first information relay points in vision. Here, photoreceptors form synapses onto two types of interneurons, bipolar cells and horizontal cells. Because outer retina synapses are particularly large and highly ordered, they have been a useful system for the discovery of mechanisms underlying synapse specificity and maintenance. Understanding these processes is critical to efforts aimed at restoring visual function through repairing or replacing neurons and promoting their connectivity. We review outer retina neuron synapse architecture, neural migration modes, and the cellular and molecular pathways that play key roles in the development and maintenance of these connections. We further discuss how these mechanisms may impact connectivity in the retina.

Early cis-regulatory events in the formation of retinal horizontal cells

During retinal development, multipotent and restricted progenitor cells generate all of the neuronal cells of the retina. Among these are horizontal cells, which are interneurons that modulate the light-induced signal from photoreceptors. This study utilizes the identification of novel cis-regulatory elements as a method to examine the gene regulatory networks that direct the development of horizontal cells. Here we describe a screen for cis-regulatory elements, or enhancers, for the horizontal cell-associated genes PTF1A, ONECUT1 (OC1), TFAP2A (AP2A), and LHX1. The OC1ECR22 and Tfap2aACR5 elements were shown to be potential enhancers for OC1 and TFAP2A, respectively, and to be specifically active in developing horizontal cells. The OC1ECR22 element is activated by PTF1A and RBPJ, which translates to regulation of OC1 expression and suggests that PTF1A is a direct activator of OC1 expression in developing horizontal cells. The region within the Tfap2aACR5 element that is responsible for its activation was determined to be a 100 bp sequence named Motif 4. Both OC1ECR22 and Tfap2aACR5 are negatively regulated by the nuclear receptors THRB and RXRG, as is the expression of OC1 and AP2A, suggesting that nuclear receptors may have a role in the negative regulation of horizontal cell development.

Loss of Crb2b-lf leads to anterior segment defects in old zebrafish

Defects in the retina or the anterior segment of the eye lead to compromised vision and affect millions of people. Understanding how these ocular structures develop and are maintained is therefore of paramount importance. The maintenance of proper vision depends, among other factors, on the function of genes controlling apico-basal polarity. In fact, mutations in polarity genes are linked to retinal degeneration in several species, including human. Here we describe a novel zebrafish crb2b allele (crb2be40 ), which specifically affects the crb2b long isoform. crb2be40 mutants are viable and display normal ocular development. However, old crb2be40 mutant fish develop multiple defects in structures of the anterior segment, which includes the cornea, the iris and the lens. Phenotypes are characterised by smaller pupils due to expansion of the iris and tissues of the iridocorneal angle, an increased number of corneal stromal keratocytes, an abnormal corneal endothelium and an expanded lens capsule. These findings illustrate a novel role for crb2b in the maintenance of the anterior segment and hence add an important function to this polarity regulator, which may be conserved in other vertebrates including humans.