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Nadean L Brown, Simranjeet Cheema, Anna La Torre; Diverse eye defects occur by varying the developmental timing of Hedgehog signaling. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4307.
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Mammalian binocular vision requires sonic Hedgehog (Shh) signaling from the ventral midline to split the eye field. Later, during neurogenesis, this pathway acts in the retina to regulate progenitor cell growth. Between these stages, the mechanisms by which Shh regulates optic cup growth, specification and patterning remain unclear, and which cells send or receive the signal for each context. To address this question, we compared the developmental timing of inducing Smoothened (Smo) loss- or gain-of-function conditionally mutant mouse embryos with different Cre drivers, then assessed the resulting phenotypes
Rax-Cre or Chx10-Cre BAC transgenics were mated to mice carrying either a Smo floxed or flox-stop SmoM2 constituitively-active alleles. Embryonic litters (E9-E13) were collected, PCR genotyped and analyzed using gross morphology, histology, immunohistochemistry and confocal microscopy or in situ hybridization, employing a battery of early brain and eye markers.
Rax-Cre initiates at E8 in the hypothalamus, optic stalk and vesicle (future retina + RPE), whereas Chx10-Cre onsets a full day later in the optic vesicle progenitor cells (future retina). We found that these Cre drivers produced dramatically different Smo gain-of-function phenotypes, from anophthalmia (Rax-Cre) to retinal neoplasia (Chx10-Cre). Rax-Cre;Smo M2/+ embryos had no optic stalk and vesicle evagination or lens induction. Yet, Chx-10Cre;Smo M2/+ embryos had dramatic optic cup and stalk overgrowth that was progressive with age. By analogy, early loss of Shh signaling causes cyclopia1, while removal of Smo activity from E9 retinal progenitors does not impact ocular morphogenesis, only retinal neurogenesis2.1Chiang et al Nature 19962Sakagami et al J Neurosi 2009
We conclude that developing optic vesicle and cup cells are sensitive to the local source(s) of Shh signaling, plus the onset and duration of its activity. Smo gain-of-function phenotypes do not effect Lhx2 expression, but induce ectopic patches of Pax6+ or Mitf+ cells in the surrounding mesenchyme, correlating with ectopic pigmented cells. We propose that too much Smo activity during ocular morphogenesis affects eye development downstream of the early eye factors Rax and Lhx2. Moreover, maintaining the appropriate amount of Shh signaling in retinal progenitor cells seems more important than blocking this activity.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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