vsx1 3' untranslated region-mediated translation difference at different developmental stages of goldfish embryos

Visual system homeobox-1 (vsx1) is important in retinal progenitor proliferation, differentiation, and function maintenance of bipolar cells in vertebrates. Recent study in Xenopus laevis has shown that vsx1 3' untranslated region (3' UTR) can mediate cell-specific translation of vsx1 mRNA in the bipolar cells of the retinal inner nuclear layer (INL). vsx1 is also transcribed at the early developmental stages prior to eye formation and its spatiotemporal expression patterns are conserved in all the examined vertebrates. In order to determine whether the vsx1 3' UTR has a role in regulating the spatiotemporal expression of vsx1 during early embryogenesis, we constructed a vsx1 UTR-controlled green fluorescent protein (GFP) reporter gene system and examined the GFP expression pattern in goldfish, Carassius auratus, at different developmental stages. Our results indicated that both the vsx1 5' UTR and the vsx1 3' UTR remarkably repressed GFP expression at transcription level but did not regulate tissue-specific translation at early developmental stages. GFP protein was ubiquitously expressed in the embryos injected with GFP-sensors containing vsx1 UTRs before 60 h post-fertilization (hpf). From hatching stage (72 hpf) onwards, however, GFP protein was specifically expressed in the bipolar cells of the retinal INL in the vsx1 3'UTR-GFP-sensor embryos, but was still ubiquitously expressed in the embryos injected with GFP-sensor lacking vsx1 3' UTR. These observations showed a significant difference of vsx1 3' UTR-mediated translation between early and late developmental stages and suggested that vsx1 3' UTR might not be involved in regulating the spatiotemporal expression of vsx1 until hatching stage during embryogenesis.