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Ross F Collery, Kerry N Veth, Brian A Link; LRP2 mediates retinoid homeostasis to regulate eye growth. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5474.
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© ARVO (1962-2015); The Authors (2016-present)
Mutations in Lrp2, a large transmembrane protein involved in receptor-mediated endocytosis and transcellular trafficking, are associated with pathological myopia in humans and zebrafish. In the eye, Lrp2 is expressed exclusively in the retinal pigment epithelium and ciliary epithelia. Lrp2 has many identified ligands, including the plasma retinol carrier, Rbp4. Since altered retinoid signaling has been implicated in experimental myopia, we explored the role of this pathway in the lrp2 mutant phenotype.
To test the role of Lrp2 in retinol trafficking, we constructed transgenic zebrafish in which Rbp4-GFP was secreted into the serum from liver hepatocytes, the endogenous source of Rbp4. Using CRISPRs to delete stra6, we tested the significance of upregulated Stra6 for the large eye phenotype of lrp2-/- mutants.
RNA transcript analysis showed altered levels of retinoic acid (RA) target genes and factors associated with retinoid homeostasis in lrp2-/- mutant eyes including rlbp1, crabp2, and stra6. Serum retinol levels were also significantly reduced compared to wild-type fish. lrp2-/- fish exposed to RA from 1-2 months of age were more sensitive to its effects than wild-type sibling controls. Importantly, lrp2-/- fish exposed to the lowest dose of RA showed ocular enlargement greater than DMSO-treated mutants, as well as RA- or DMSO-treated wild-type fish. Furthermore, lrp2-/- mutants showed genetic interaction with mutants for cyp26a1, a cytochrome P450-type enzyme that degrades RA. In humans, cyp26a1 variants have been shown to be associated with myopia. Our genetic experiments showed that lrp2-/-;cyp26a1+/- animals exhibited exacerbated myopia compared to lrp2-/- mutant fish alone. Rbp4-GFP accumulated in the sclera and choroid layers of lrp2-/- mutant eyes, but not in those of wild-type sibling fish. In vivo fluorescent recovery after photobleaching analysis indicated that loss of Lrp2 prevented normal Rbp4-GFP uptake into RPE cells. lrp2-/- stra6-/- double mutant fish were partially rescued for the myopic phenotype compared to lrp2-/- single mutants.
These and other results suggest that RPE cells modulate emmetropization in part by controlling homeostasis of periocular retinoids, and dysregulation contributes high myopia. This process is primarily mediated through Lrp2, and its loss results in accumulation of periocular retinol-RBP4 (the precursor for RA) and enhanced import (via upregulated Stra6).
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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