Foxq2 determines blue cone identity in zebrafish

Abstract

In vertebrates, daylight vision is mediated by a combination of spectrally distinct cone photoreceptor cells. Most vertebrate lineages retain a tetrachromatic cone system in which each cone photoreceptor subtype expresses one of four cone opsins: UV- (SWS1), blue- (SWS2), green- (RH2), and red-sensitive (LWS) opsins. Each cone subtype identity is established by a transcriptional network directing selective opsin gene expression in single photoreceptors. Our knowledge is limited regarding gene expression mechanisms for the middle wavelength-sensitive opsin genes, sws2 and rh2, because they are absent in mammalian species such as mouse, whose visual system has been extensively studied. Our previous studies identified homeobox transcription factors, Six6 and Six7, as crucial regulators of both sws2 and rh2 gene expression in zebrafish. Yet it remains unclear how these two opsin genes are selectively expressed in a cone subtype-specific manner. Here we pursued loss-of-function studies on transcription factors expressed predominantly in zebrafish cone photoreceptors and found that sws2 expression requires a forkhead box transcription factor, Foxq2, which is retained in many vertebrates having sws2 gene. A quantitative gene expression analysis using purified pools of the four cone subtypes revealed that foxq2 was expressed only in SWS2 cone subtype. foxq2 expression was abrogated in six6a/six6b/six7 knock-out zebrafish, which is deficient in SWS2 cone subtype. Forced expression of foxq2 fully restored sws2 expression in six7 knock-out fish without affecting rh2 expression. We propose a core transcriptional network that determines SWS2 cone subtype identity in the tetrachromatic vertebrate.

Partitioning of gene expression among zebrafish photoreceptor subtypes

“…Our analysis also implicated 22 transcriptional regulators in the control of cone subtype-specific gene expression. Known regulators such as tbx2b , foxq2 , and thrb show the strongest positive associations with UV, blue, and red cone-specific marker genes, respectively 30 , 31 , 34 . We also discovered novel candidate regulators of UV cone genes ( tbx2a ), UV and blue cone genes ( skor1a ), and red cone genes ( sox6 and glis3 ).…”

“…S1, and Supplementary Data S1 ). These genes include subtype-defining opsin genes as well as previously identified subtype markers, such as rom1a (rod) 42 , foxq2 (blue) 31 , thrb (red) 31 , and si:busm1–57f23.1 (red) 33 . We also identified two pre-microRNAs (mir729 and mir726) expressed exclusively in UV and red cones, respectively, similar to what was previously described in medaka ( Oryzias latipes ) 43 (Fig.…”

“…Whereas others are more specifically involved in rod (RORB, NRL, NR2E3, ESRRB, CASZ1, and SAMD7) 22 – 27 or cone development and/or function (THRB, RXRG, RORA, COUP-TFI/COUP-TFII, and OC1/OC2) 28 , 29 . In zebrafish, studies have begun to identify additional transcription factors required for the development of specific cone subtypes: tbx2b in UV cones 30 , foxq2 in blue cones 31 , six6a , six6b , and six7 in blue and green cones 32 , and thrb in red cones 33 , 34 . Despite these advances, the architecture of the transcriptional regulatory networks that govern photoreceptor diversification in zebrafish remain largely unknown.…”