Abstract
Midkine is a heparin binding growth factor with important functions in neuronal development and survival, but little is known about its function in the retina. Previous studies show that in the developing zebrafish, Midkine-a (Mdka) regulates cell cycle kinetics in retinal progenitors, and following injury to the adult zebrafish retina, mdka is strongly upregulated in Müller glia and the injury-induced photoreceptor progenitors. Here we provide the first data describing Mdka protein localization during different stages of retinal development and during the regeneration of photoreceptors in adults. We also experimentally test the role of Mdka during photoreceptor regeneration. The immuno-localization of Mdka reflects the complex spatiotemporal pattern of gene expression and also reveals the apparent secretion and extracellular trafficking of this protein. During embryonic retinal development the Mdka antibodies label all mitotically active cells, but at the onset of neuronal differentiation, immunostaining is also localized to the nascent inner plexiform layer. Starting at five days post fertilization through the juvenile stage, Mdka immunostaining labels the cytoplasm of horizontal cells and the overlying somata of rod photoreceptors. Double immunolabeling shows that in adult horizontal cells, Mdka co-localizes with markers of the Golgi complex. Together, these data are interpreted to show that Mdka is synthesized in horizontal cells and secreted into the outer nuclear layer. In adults, Mdka is also present in the end feet of Müller glia. Similar to mdka gene expression, Mdka in horizontal cells is regulated by circadian rhythms. After the light-induced death of photoreceptors, Mdka immuonolabeling is localized to Müller glia, the intrinsic stem cells of the zebrafish retina, and proliferating photoreceptor progenitors. Knockdown of Mdka during photoreceptor regeneration results in less proliferation and diminished regeneration of rod photoreceptors. These data suggest that during photoreceptor regeneration Mdka regulates aspects of injury-induced cell proliferation.
Highlights
Midkine is a heparin-binding growth factor that forms a two-member family with Pleiotrophin
Cellular localization of Mdka during retinal development and in the adult retina To test the specificity of the Mdka antibodies, detection of Mdka and its absence in mdka morphants was confirmed using Western blots, the antibodies were used for immunofluorescence on retinal cryosections taken from control and morphant embryos
The zebrafish retina is formed from a pool of mitotic progenitors that begin exiting the cell cycle around 28–32 hpf [60,61] and thereafter continue to exit the cell cycle in a complex spatiotemporal pattern [62]
Summary
Midkine is a heparin-binding growth factor that forms a two-member family with Pleiotrophin. Both factors are abundantly expressed during embryogenesis, with high levels in the developing nervous system [1]. Beyond mid-gestation and during postnatal stages, the expression of midkine and pleiotrophin are rapidly downregulated [2,3,4,5,6]. Genes encoding both Midkine and Pleiotrophin are up-regulated under disease conditions, most notably those that affect the nervous system [7,8,9,10,11]. Throughout the nervous system Midkine is proposed to play a role in reparative mechanisms
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