Capicua regulates the development of adult-born neurons in the hippocampus.

New neurons continuously arise from neural progenitor cells in the dentate gyrus of the adult hippocampus to support ongoing learning and memory formation. To generate functional adult-born neurons, neural progenitor cells proliferate to expand the precursor cell pool and differentiate into neurons. Newly generated cells then undergo postmitotic maturation to migrate to their final destination and develop elaborate dendritic branching, which allows them to receive input signals. Little is known about factors that regulate neuronal differentiation, migration, and dendrite maturation during adult hippocampal neurogenesis. Here, we show that the transcriptional repressor protein capicua (CIC) exhibits dynamic expression in the adult dentate gyrus. Conditional deletion of Cic from the mouse dentate gyrus compromises the adult neural progenitor cell pool without altering their proliferative potential. We further demonstrate that the loss of Cic impedes neuronal lineage development and disrupts dendritic arborization and migration of adult-born neurons. Our study uncovers a previously unrecognized role of CIC in neurogenesis of the adult dentate gyrus.

The expression, localisation and interactome of pigeon CRY2.

Cryptochromes (CRY) are highly conserved signalling molecules that regulate circadian rhythms and are candidate radical pair based magnetoreceptors. Birds have at least four cryptochromes (CRY1a, CRY1b, CRY2, and CRY4), but few studies have interrogated their function. Here we investigate the expression, localisation and interactome of clCRY2 in the pigeon retina. We report that clCRY2 has two distinct transcript variants, clCRY2a, and a previously unreported splice isoform, clCRY2b which is larger in size. We show that clCRY2a mRNA is expressed in all retinal layers and clCRY2b is enriched in the inner and outer nuclear layer. To define the localisation and interaction network of clCRY2 we generated and validated a monoclonal antibody that detects both clCRY2 isoforms. Immunohistochemical studies revealed that clCRY2a/b is present in all retinal layers and is enriched in the outer limiting membrane and outer plexiform layer. Proteomic analysis showed clCRY2a/b interacts with typical circadian molecules (PER2, CLOCK, ARTNL), cell junction proteins (CTNNA1, CTNNA2) and components associated with the microtubule motor dynein (DYNC1LI2, DCTN1, DCTN2, DCTN3) within the retina. Collectively these data show that clCRY2 is a component of the avian circadian clock and unexpectedly associates with the microtubule cytoskeleton.

Analysis of zebrafish cryptochrome2 and 4 expression in UV cone photoreceptors.

Cryptochromes (Cry) are ancient flavoproteins known to regulate circadian rhythms. In plants and some animals, Cry is sensitive to blue light due to its ability to bind the chromophore flavin adenine dinucleotide (FAD). Cry is also suggested to function in magnetoreception, since it can create light-dependent radical pairs with FAD that are sensitive to magnetic fields (Ritz2000; Liedvogel et al., 2007; Solov'yov et al., 2014). Cry is expressed in the visual system of various animals and specifically co-localizes with both short- and long-wavelength cone photoreceptors in birds (Bischof et al., 2011; Günther et al., 2018). However, magnetoreception is not limited to birds and the expression of cry genes in the photoreceptors of other vertebrates is unknown. Here, we use zebrafish to examine the retinal expression pattern of cry family genes. Zebrafish have seven cry genes and while most are known regulators of the circadian clock, relatively little is known about cry2 and cry4 (Haug et al., 2015; Liu et al., 2015). Therefore, we explored cry2 and cry4 expression in the larval and adult zebrafish retina. We demonstrate that cry4 is predominantly expressed in the short-wavelength ultraviolet (UV)-sensitive cone photoreceptors, while cry2 is expressed in UV cones and additional retinal photoreceptors during the day. Using Nitroreductase (NTZ)-mediated cell ablation and qRT-PCR, we find that cry4 expression significantly decreases when UV cones are ablated, but not when the neighboring short-wavelength sensitive blue cones are ablated. cry2 expression decreases after UV cone ablation but is still significantly detectable, while blue cone ablation does not alter cry2 expression. This study provides a more detailed annotation of cry2 and cry4 expression in the zebrafish retina and highlights the feasibility of a well-established ablation paradigm to test if photoreceptors are required for magnetoreception in fish. Although evidence of magnetoreception in adult zebrafish has gained considerable evidence over the last decade (Shcherbakov et al., 2005; Takebe et al., 2012; Krylov et al., 2016; Myklatun et al., 2018) the mediating mechanism(s) remain unknown. Additionally, despite limited evidence that larval zebrafish are magnetoreceptive, many other larval fish have a characterized magnetic sense; sockeye salmon fry, larval coral reef fish, larval medaka and larval Atlantic haddock have been shown to be responsive to magnetic fields (Quinn; 1980; Bottesch et al., 2016; O'Connor and Muheim. 2017; Myklatun et al., 2018; Cresci, et al. 2019). If cry-cone interactions are conserved within fish, our findings may suggest one potential mechanism, such that UV cones appear poised for light-dependent magnetoreception via photoreceptor subtype-specific expression of cry.

Nrl Is Dispensable for Specification of Rod Photoreceptors in Adult Zebrafish Despite Its Deeply Conserved Requirement Earlier in Ontogeny.

The transcription factor NRL (neural retina leucine zipper) has been canonized as the master regulator of photoreceptor cell fate in the retina. NRL is necessary and sufficient to specify rod cell fate and to preclude cone cell fate in mice. By engineering zebrafish, we tested if NRL function has conserved roles beyond mammals or beyond nocturnal species, i.e., in a vertebrate possessing a greater and more typical diversity of cone sub-types. Transgenic expression of Nrl from zebrafish or mouse was sufficient to induce rod photoreceptor cells. Zebrafish nrl -/- mutants lacked rods (and had excess UV-sensitive cones) as young larvae; thus, the conservation of Nrl function between mice and zebrafish appears sound. Strikingly, however, rods were abundant in adult nrl -/- null mutant zebrafish. Rods developed in adults despite Nrl protein being undetectable. Therefore, a yet-to-be-revealed non-canonical pathway independent of Nrl is able to specify the fate of some rod photoreceptors.

Abnormal Cone and Rod Photoreceptor Morphogenesis in gdf6a Mutant Zebrafish.

Purpose: Analysis of photoreceptor morphology and gene expression in mispatterned eyes of zebrafish growth differentiation factor 6a (gdf6a) mutants. Methods: Rod and cone photoreceptors were compared between gdf6a mutant and control zebrafish from larval to late adult stages using transgenic labels, immunofluorescence, and confocal microscopy, as well as by transmission electron microscopy. To compare transcriptomes between larval gdf6a mutant and control zebrafish, RNA-Seq was performed on isolated eyes. Results: Although rod and cone photoreceptors differentiate in gdf6a mutant zebrafish, the cells display aberrant growth and morphology. The cone outer segments, the light-detecting sensory endings, are reduced in size in the mutant larvae and fail to recover to control size at subsequent stages. In contrast, rods form temporarily expanded outer segments. The inner segments, which generate the required energy and proteins for the outer segments, are shortened in both rods and cones at all stages. RNA-Seq analysis provides a set of misregulated genes associated with the observed abnormal photoreceptor morphogenesis. Conclusions: GDF6 mutations were previously identified in patients with Leber congenital amaurosis. Here, we reveal a unique photoreceptor phenotype in the gdf6a mutant zebrafish whereby rods and cones undergo abnormal maturation distinct for each cell type. Further, subsequent development shows partial recovery of cell morphology and maintenance of the photoreceptor layer. By conducting a transcriptomic analysis of the gdf6a larval eyes, we identified a collection of genes that are candidate regulators of photoreceptor size and morphology.