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The cellular expression of midkine-a and midkine-b during retinal development and photoreceptor regeneration

ABSTRACT: In the retina of adult teleosts, stem cells are sustained in two specialized niches: the ciliary marginal zone (CMZ) and the microenvironment surrounding adult Müller glia. Recently, Müller glia were identified as the regenerative stem cells in the teleost retina. Secreted signaling molecules that regulate neuronal regeneration in the retina are largely unknown. In a microarray screen to discover such factors, we identified midkine-b (mdkb). Midkine is a highly conserved heparin-binding growth factor with numerous biological functions. The zebrafish genome encodes two distinct midkine genes: mdka and mdkb. Here, we describe the cellular expression of mdka and mdkb during retinal development and the initial, proliferative phase of photoreceptor regeneration. The results show that in the embryonic and larval retina mdka and mdkb are expressed in stem cells, retinal progenitors and neurons in distinct patterns that suggest different functions for the two molecules. Following the selective death of photoreceptors in the adult, mdka and mdkb are co-expressed in horizontal cells and proliferating Müller glia and their neurogenic progeny. These data reveal that Mdka and Mdkb are signaling factors present in the retinal stem cell niches in both embryonic and mature retinas, and that their cellular expression is actively modulated during retinal development and regeneration.

ORGANISM(S): Danio rerio

PROVIDER: GSE13999 | GEO | 2008/12/17

SECONDARY ACCESSION(S): PRJNA112499

REPOSITORIES: GEO

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Project description:Loss of neurons in the neural retina is a leading cause of vision loss. Retinal regeneration in zebrafish is attributed to Müller glia, which are activated, adopt a stem cell-like state, proliferate, and generate new neurons. Despite the same endogenous Müller glia being present in all vertebrates including humans, Müller glia activation in mammals leads to glial scarring instead of neurogenesis. Therefore, understanding Müller glia cellular states and molecular processes during zebrafish regeneration may allow us to improve mammalian regeneration. Since only a subset of Müller glia activate following neuron loss, we asked whether a specialised subset of Müller glia might be genetically primed for activation. Single-cell RNA sequencing (sc-RNAseq) from integrated zebrafish Müller glia samples reveals heterogeneity of gene expression across the quiescent Müller glia pool with 4 main clusters emerging, two of which are genetically similar. Labelling for key differentially expressed markers identified that these main clusters correlated with glia spatially distributed across dorsal, central and ventral retina. Using a genetically driven, chemically induced nitroreductase approach, three distinct photoreceptor types were ablated: the long (Lws2), short wavelength-sensitive (Sws2) and rod (Xops) photoreceptors, which also differ in their abundance. As expected, histological analysis of the three injuries and sc-RNAseq data for the two cone photoreceptor ablations identified common genetic program involved in the transition of Müller glia quiescence to activation, and differential responses to injury related to either injury extent or subtype of photoreceptor targeted. Interestingly, despite differences in the distributed across the retina, biased activation of Müller glia was observed in dorsal and central regions. Gene ontology analysis revealed that these injury-responsive dorsal and central Müller glia express genes related to dorsal/ventral pattern formation, growth factor activity, and regulation of developmental process. Müller glia upregulated genes related to homeostasis as well as certain AP-1 and injury-responsive transcription factors, followed by expression of genes involved in cell cycle, chromatin remodeling, and microtubule organisation. Prior to cell cycle entry, Müller glia show a transient upregulation of genes involved in gliogenesis, and Notch signalling. These findings enhance our understanding of heterogeneous states of quiescent Müller glia, and how these differences relate to activation and regeneration potential following neural ablation. A comparison of the distinct quiescent Müller glia with glia states in mammals will reveal key molecular pathways that could be targeted for improved mammalian neural regeneration.

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The cellular expression of midkine-a and midkine-b during retinal development and photoreceptor regeneration

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