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J. Luo, A.-A. Calinescu, P. F. Hitchcock; Midkine-a and Midkine-b Regulates Neurogenesis and Neural Differentiation in the Retina of Zebrafish. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4001.
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Midkine is a small secreted heparin binding growth factor that has numerous biological functions, both during development and following tissue injury. Previously, we identified midkine as a gene induced in the retina of the zebrafish following photoreceptor death and during photoreceptor regeneration. There are two midkine orthologs in zebrafish, midkine-a (mdka) and midkine-b (mdkb). The purpose of this study was to use the embryonic zebrafish to experimentally investigate function of Mdka during retinal neurogensis.
RNA in situ hybridizationwas used to determine the cellular expression of mdka, mdkb and the receptor-type protein tyrosine phosphatase-zeta (rPTPζ), a putative component of the midkine receptor. Morpholino oligonucletides were injected into early-stage embryos to knock down mdka synthesis, which was verified by Western analysis. Embryos were soaked in BrdU to systemically label proliferating cells. Immunocytochemistry was then used to identify proliferating cells, and other antibody markers were used to identify specific cell types. Retinas from experimental and control animals were evaluated at 48, 72 and 120 hours post fertilization (hpf).
In the embryonic and larval retina, mdka is expressed in the retinal stem and progenitor cells, whereas rPTPζ is expressed by differentiated neurons within the inner nuclear layer. During embryonic and larval development, blocking mdka synthesis does not alter gross body or eye development. However, at both 48 and 72 hpf, knocking down mdka synthesis alters the proliferation of retinal progenitors, the differentiation of retinal neurons and the cellular expression of rPTPζ. At 120 hpf, approximately 2 days after the time when morpholino-dependent translation inhibition is lost, the morpholino-dependent alterations in retinal development are lost and normal retinal development recovers.
In the embryonic and larval retina of zebrafish, mdka is a molecular component of the stem cell niche and during early retinal development functions as a regulator of neurogenesis and neuronal differentiation. This study provides insights into a potentially novel signaling pathway during both retinal development and photoreceptor regeneration.
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