Generating Retinal Injury Models in <em>Xenopus</em> Tadpoles

Retinal neurodegenerative diseases are the leading causes of blindness. Among numerous therapeutic strategies being explored, stimulating self-repair ...

Université Paris-Saclay

Usher syndrome type 1 (USH1) causes combined hearing and sight defects, but how mutations in USH1 genes lead to retinal dystrophy in patients remains elusive. The USH1 protein complex is associated with calyceal processes, which are microvilli of unknown function surrounding the base of the photoreceptor outer segment. We show that in , these processes are connected to the outer-segment membrane by links composed of protocadherin-15 (USH1F protein). Protocadherin-15 deficiency, obtained by a knockdown approach, leads to impaired photoreceptor function and abnormally shaped photoreceptor outer segments. Rod basal outer disks displayed excessive outgrowth, and cone outer segments were curved, with lamellae of heterogeneous sizes, defects also observed upon knockdown of , encoding cadherin-23 (USH1D protein). The calyceal processes were virtually absent in cones and displayed markedly reduced F-actin content in rods, suggesting that protocadherin-15-containing links are essential for their development and/or maintenance. We propose that calyceal processes, together with their associated links, control the sizing of rod disks and cone lamellae throughout their daily renewal.

Usher syndrome type 1-associated cadherins shape the photoreceptor outer segment.

Amacrine interneurons that modulate synaptic plasticity between bipolar and ganglion cells constitute the most diverse cell type in the retina. Most are inhibitory neurons using either GABA or glycine as neurotransmitters. Although several transcription factors involved in amacrine cell fate determination have been identified, mechanisms underlying amacrine cell subtype specification remain to be further understood. The Prdm13 histone methyltransferase encoding gene is a target of the transcription factor Ptf1a, an essential regulator of inhibitory neuron cell fate in the retina. Here, we have deepened our knowledge on its interaction with Ptf1a and investigated its role in amacrine cell subtype determination in the developing Xenopus retina.

Prdm13 forms a feedback loop with Ptf1a and is required for glycinergic amacrine cell genesis in the Xenopus Retina.

Wnt proteins form a family of highly conserved secreted molecules that are critical mediators of cell-cell signaling during embryogenesis. Partial data on Wnt activity in different tissues and at different stages have been reported in frog embryos. Our objective here is to provide a coherent and detailed description of Wnt activity throughout embryo development. Using a transgenic Xenopus tropicalis line carrying a Wnt-responsive reporter sequence, we depict the spatial and temporal dynamics of canonical Wnt activity during embryogenesis. We provide a comprehensive series of in situ hybridization in whole-mount embryos and in cross-sections, from gastrula to tadpole stages, with special focus on neural tube, retina and neural crest cell development. This collection of patterns will thus constitute a valuable resource for developmental biologists to picture the dynamics of Wnt activity during development.

An atlas of Wnt activity during embryogenesis in Xenopus tropicalis.

A hallmark of Wnt/β-Catenin signaling is the extreme diversity of its transcriptional response, which varies depending on the cell and developmental context. What controls this diversity is poorly understood. In all cases, the switch from transcriptional repression to activation depends on a nuclear increase in β-Catenin, which detaches the transcription factor T cell factor 7 like 1 (Tcf7l1) bound to Groucho (Gro) transcriptional co-repressors from its DNA-binding sites and transiently converts Tcf7/Lymphoid enhancer binding factor 1 (Lef1) into a transcriptional activator. One of the earliest and evolutionarily conserved functions of Wnt/β-Catenin signaling is the induction of the blastopore lip organizer. Here, we demonstrate that the evolutionarily conserved BarH-like homeobox-2 (Barhl2) protein stabilizes the Tcf7l1-Gro complex and maintains the repressed expression of Tcf target genes by a mechanism that depends on histone deacetylase 1 (Hdac-1) activity. In this way, Barhl2 switches off the Wnt/β-Catenin-dependent early transcriptional response, thereby limiting the formation of the organizer in time and/or space. This study reveals a novel nuclear inhibitory mechanism of Wnt/Tcf signaling that switches off organizer fate determination.

Barhl2 maintains T cell factors as repressors and thereby switches off the Wnt/β-Catenin response driving Spemann organizer formation.

Retinitis pigmentosa is an inherited retinal dystrophy that ultimately leads to blindness due to the progressive degeneration of rod photoreceptors and the subsequent non-cell autonomous death of cones.  is the most frequently mutated gene in this disease. We here developed  gene editing-based models of retinitis pigmentosa in two  species,  and , by using CRISPR/Cas9 technology. In both of them, loss of  function results in massive rod cell degeneration characterized by progressive shortening of outer segments and occasional cell death. This is followed by cone morphology deterioration. Despite these apparently similar degenerative environments, we found that Müller glial cells behave differently in  and . While a significant proportion of Müller cells re-enter into the cell cycle in , their proliferation remains extremely limited in . This work thus reveals divergent responses to retinal injury in closely related species. These models should help in the future to deepen our understanding of the mechanisms that have shaped regeneration during evolution, with tremendous differences across vertebrates.

Karine Parain

Paris-Saclay Institute of Neuroscience,
CNRS,
Paris-Saclay Institute of Neuroscience, CNRS

Université Paris-Saclay

Generating Retinal Injury Models in Xenopus Tadpoles

Karine Parain

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Université Paris-Saclay

Generating Retinal Injury Models in Xenopus Tadpoles

Paris-Saclay Institute of Neuroscience,
CNRS,
Paris-Saclay Institute of Neuroscience, CNRS