Retinal Expression of Wnt-Pathway Mediated Genes in Low-Density Lipoprotein Receptor-Related Protein 5 (Lrp5) Knockout Mice

Abstract

Mutations in low-density lipoprotein receptor-related protein 5 (Lrp5) impair retinal angiogenesis in patients with familial exudative vitreoretinopathy (FEVR), a rare type of blinding vascular eye disease. The defective retinal vasculature phenotype in human FEVR patients is recapitulated in Lrp5 knockout (Lrp5−/−) mouse with delayed and incomplete development of retinal vessels. In this study we examined gene expression changes in the developing Lrp5−/− mouse retina to gain insight into the molecular mechanisms that underlie the pathology of FEVR in humans. Gene expression levels were assessed with an Illumina microarray on total RNA from Lrp5−/− and WT retinas isolated on postnatal day (P) 8. Regulated genes were confirmed using RT-qPCR analysis. Consistent with a role in vascular development, we identified expression changes in genes involved in cell-cell adhesion, blood vessel morphogenesis and membrane transport in Lrp5−/− retina compared to WT retina. In particular, tight junction protein claudin5 and amino acid transporter slc38a5 are both highly down-regulated in Lrp5−/− retina. Similarly, several Wnt ligands including Wnt7b show decreased expression levels. Plasmalemma vesicle associated protein (plvap), an endothelial permeability marker, in contrast, is up-regulated consistent with increased permeability in Lrp5−/− retinas. Together these data suggest that Lrp5 regulates multiple groups of genes that influence retinal angiogenesis and may contribute to the pathogenesis of FEVR.

Pharmacologic Activation of Wnt Signaling by Lithium Normalizes Retinal Vasculature in a Murine Model of Familial Exudative Vitreoretinopathy

“…Mice deficient in Lrp5, Fzd4, Norrin, or Tspan12 all display similar retinal vascular phenotypes, including delayed vascular development, persistent hyaloid vasculature, lack of inner retinal vasculature, and impaired visual function. 6,17,49 Although genetic overexpression of Norrin 52,53 or stabilization of b-catenin 40 were found to be effective in rescuing vascular defects in Norrin-deficient mice, no pharmacologic treatments have yet been identified for FEVR or Norrie disease. Future investigations will determine whether lithium treatment is also effective in genetic models of FEVR other than Lrp5 À/À mice.…”

“…After birth, retinal vessels in mice grow radially from the optic disk and reach peripheral retina by P7 to P8. 11,17 Lrp5 À/À mice displayed significantly less retinal vascular coverage and fewer branching points in the central superficial vascular plexus at P7 (Figure 1), compared with age-matched WT mice. Daily treatment with lithium (P1 to P7) accelerated retinal vascular growth, modestly improved superficial plexus vascular coverage (approximately 7.6% more than vehicle control), and substantially increased the number of branching points (approximately 42% more than vehicle control) at P7 compared with vehicle-treated Lrp5 À/À littermates (Figure 1), indicating a proangiogenic role of lithium in the development of superficial vascular plexus in Lrp5 À/À eyes.…”

“…8,12,15,16 In FEVR, lack of any one of LRP5/Frizzled4/TSPAN12/ Norrin causes deficient canonical Wnt signaling pathway at the ligand/receptor level, which allows the GSK3 complex to increase b-catenin degradation, resulting in eventual suppression of physiological vessel growth in the retina. 2,17 Both Sox17 and claudin5 were down-regulated in Wnt-deficient retinal vessels and were suggested to play a potential role in mediating vascular formation in FEVR retina, 11,18 although the exact underlying molecular mechanisms of Wnt signalingemediated retinal vascular defects are not completely understood yet. To date, there is no effective treatment for FEVR.…”

“…Knockout of Lrp5 (Lrp5 À/À ) in mice produces eye vascular pathologies that model FEVR in humans, 17,21 whereas heterozygous mice are largely phenotypically normal. 17 Here, we fully characterized the development of ocular vascular pathologies, visual functional deficiency, in vivo eye measurements, and defective Wnt activation in Lrp5 À/À eyes.…”

“…17 Here, we fully characterized the development of ocular vascular pathologies, visual functional deficiency, in vivo eye measurements, and defective Wnt activation in Lrp5 À/À eyes. Moreover, we hypothesized that one potential approach to treat FEVR is through bypassing the genetic defects at the receptor level and activating the Wnt signaling pathway downstream with the use of a Wnt activator lithium that inhibits GSK3b.…”