No abstract

Summary The black nectar produced by Melianthus flowers is thought to serve as a visual attractant to bird pollinators, but the chemical identity and synthesis of the black pigment are unknown. A combination of analytical biochemistry, transcriptomics, proteomics, and enzyme assays was used to identify the pigment that gives Melianthus nectar its black color and how it is synthesized. Visual modeling of pollinators was also used to infer a potential function of the black coloration. High concentrations of ellagic acid and iron give the nectar its dark black color, which can be recapitulated through synthetic solutions containing only ellagic acid and iron(iii). The nectar also contains a peroxidase that oxidizes gallic acid to form ellagic acid. In vitro reactions containing the nectar peroxidase, gallic acid, hydrogen peroxide, and iron(iii) fully recreate the black color of the nectar. Visual modeling indicates that the black color is highly conspicuous to avian pollinators within the context of the flower. Melianthus nectar contains a natural analog of iron‐gall ink, which humans have used since at least medieval times. This pigment is derived from an ellagic acid‐Fe complex synthesized in the nectar and is likely involved in the attraction of passerine pollinators endemic to southern Africa.

In mammals, photoreceptor loss causes permanent blindness, but in zebrafish (Danio rerio), Müller glia function as intrinsic stem cells, producing progenitor cells that regenerate photoreceptors and restore vision. MicroRNAs (miRNAs) critically regulate neurogenesis in the brain and retina, but the roles of miRNAs in injury-induced neuronal regeneration are largely unknown. The miRNA miR-18a regulates photoreceptor differentiation in the embryonic retina. The purpose of the current study was to determine the function of miR-18a during injury-induced photoreceptor regeneration. RT-qPCR, in-situ hybridization (ISH) and immunohistochemistry (IHC) showed that miR-18a expression increases throughout the retina by 1-day post-injury (dpi) and continues to increase through 5 dpi. Bromodeoxyuridine (BrdU) labeling showed that at 7 and 10 dpi, when regenerated photoreceptors are normally differentiating, there are more proliferating Müller glia-derived progenitors in homozygous miR-18a mutant (miR-18ami5012) retinas compared with wild type (WT), indicating that miR-18a negatively regulates injury-induced proliferation. At 7 and 10 dpi, miR-18ami5012 retinas have fewer mature photoreceptors than WT, but there is no difference at 14 dpi, revealing that photoreceptor regeneration is delayed. BrdU labeling showed that the excess progenitors in miR-18ami5012 retinas migrate to other retinal layers besides the photoreceptor layer. Inflammation is critical for photoreceptor regeneration and RT-qPCR showed that, in the absence of miR-18a, inflammation is prolonged. Suppressing inflammation with dexamethasone rescues the miR-18ami5012 phenotype. Together, these data show that during injury-induced photoreceptor regeneration, miR-18a regulates proliferation and photoreceptor regeneration by regulating key aspects of the inflammatory response during photoreceptor regeneration in zebrafish.

In mammals, photoreceptor loss causes permanent blindness, but in zebrafish (Danio rerio), Müller glia function as intrinsic stem cells, producing progenitor cells that regenerate photoreceptors and restore vision. MicroRNAs (miRNAs) critically regulate neurogenesis in the brain and retina, but the roles of miRNAs in injury-induced neuronal regeneration are largely unknown. The miRNA miR-18a regulates photoreceptor differentiation in the embryonic retina. The purpose of the current study was to determine the function of miR-18a during injury-induced photoreceptor regeneration. RT-qPCR, in-situ hybridization (ISH) and immunohistochemistry (IHC) showed that miR-18a expression increases throughout the retina by 1-day post-injury (dpi) and continues to increase through 5 dpi. Bromodeoxyuridine (BrdU) labeling showed that at 7 and 10 dpi, when regenerated photoreceptors are normally differentiating, there are more proliferating Müller glia-derived progenitors in homozygous miR-18a mutant (miR-18ami5012) retinas compared with wild type (WT), indicating that miR-18a negatively regulates injury-induced proliferation. At 7 and 10 dpi, miR-18ami5012 retinas have fewer mature photoreceptors than WT, but there is no difference at 14 dpi, revealing that photoreceptor regeneration is delayed. BrdU labeling showed that the excess progenitors in miR-18ami5012 retinas migrate to other retinal layers besides the photoreceptor layer. Inflammation is critical for photoreceptor regeneration and RT-qPCR showed that, in the absence of miR-18a, inflammation is prolonged. Suppressing inflammation with dexamethasone rescues the miR-18ami5012 phenotype. Together, these data show that during injury-induced photoreceptor regeneration, miR-18a regulates proliferation and photoreceptor regeneration by regulating key aspects of the inflammatory response during photoreceptor regeneration in zebrafish.

In mammals, photoreceptor loss causes permanent blindness, but in zebrafish (Danio rerio), photoreceptor loss reprograms Müller glia to function as stem cells, producing progenitors that fully regenerate photoreceptors. MicroRNAs (miRNAs) regulate neurogenesis in the CNS, but the roles of miRNAs in injury-induced neuronal regeneration are largely unknown. In the embryonic zebrafish retina, miRNA miR-18a regulates photoreceptor differentiation. The purpose of the current study was to determine in zebrafish the function of miR-18a during injury-induced photoreceptor regeneration. RT-qPCR, in-situ hybridization and immunohistochemistry showed that miR-18a expression increases throughout the retina by 1-day post-injury (dpi) and increases through 5 dpi. To test miR-18a function during photoreceptor regeneration, we used homozygous miR-18a mutants (miR-18ami5012), and knocked down miR-18a with morpholino oligonucleotides. During photoreceptor regeneration, miR-18ami5012 retinas have fewer mature photoreceptors than WT at 7 and 10 dpi, but there is no difference at 14 dpi, indicating that photoreceptor regeneration is delayed. Labeling dividing cells with bromodeoxyuridine (BrdU) showed that at 7 and 10 dpi, there are excess Müller glia-derived progenitors in both mutants and morphants, indicating that miR-18a negatively regulates injury-induced proliferation. Tracing BrdU-labeled cells showed that in miR-18ami5012 retinas excess progenitors migrate to other retinal layers in addition to the photoreceptor layer. Inflammation is critical for photoreceptor regeneration, and RT-qPCR showed that in miR-18ami5012 retinas, inflammatory gene expression and microglia activation are prolonged. Suppressing inflammation with dexamethasone rescues the miR-18ami5012 phenotype. Together, these data show that during photoreceptor regeneration in zebrafish, miR-18a regulates proliferation and photoreceptor regeneration by regulating the inflammatory response.