TSPAN12 Regulates Retinal Vascular Development by Promoting Norrin- but Not Wnt-Induced FZD4/β-Catenin Signaling

Abstract

Mutations in the genes encoding the Wnt receptor Frizzled-4 (FZD4), coreceptor LRP5, or the ligand Norrin disrupt retinal vascular development and cause ophthalmic diseases. Although Norrin is structurally unrelated to Wnts, it binds FZD4 and activates the canonical Wnt pathway. Here we show that the tetraspanin Tspan12 is expressed in the retinal vasculature, and loss of Tspan12 phenocopies defects seen in Fzd4, Lrp5, and Norrin mutant mice. In addition, Tspan12 genetically interacts with Norrin or Lrp5. Overexpressed TSPAN12 associates with the Norrin-receptor complex and significantly increases Norrin/beta-catenin but not Wnt/beta-catenin signaling, whereas Tspan12 siRNA abolishes transcriptional responses to Norrin but not Wnt3A in retinal endothelial cells. Signaling defects caused by Norrin or FZD4 mutations that are predicted to impair receptor multimerization are rescued by overexpression of TSPAN12. Our data indicate that Norrin multimers and TSPAN12 cooperatively promote multimerization of FZD4 and its associated proteins to elicit physiological levels of signaling.

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.…”

The TspanC8 Subgroup of Tetraspanins Interacts with A Disintegrin and Metalloprotease 10 (ADAM10) and Regulates Its Maturation and Cell Surface Expression

“…Moreover, Tspan12 was proposed as an ADAM10-interacting tetraspanin (24). However, because ADAM10 appears to be ubiquitous but Tspan12 is not (16), it seemed likely that other tetraspanins would interact with ADAM10. Analyses of the Tspan12-deficient mouse did not address a potential role in ADAM10 regulation but instead demonstrated a role for Tspan12 in the regulation of Frizzled-4-induced ␤-catenin signaling (16).…”

“…However, because ADAM10 appears to be ubiquitous but Tspan12 is not (16), it seemed likely that other tetraspanins would interact with ADAM10. Analyses of the Tspan12-deficient mouse did not address a potential role in ADAM10 regulation but instead demonstrated a role for Tspan12 in the regulation of Frizzled-4-induced ␤-catenin signaling (16). Tspan12 deficiency leads to defective vascular development in the retina, and Tspan12 mutations cause the human blinding disease familial exudative vitreoretinopathy, thus phenocopying mutations in the Frizzled-4 signaling pathway (17,18).…”

“…Moreover, ADAM10 activity was enhanced by Tspan12 as detected by shedding of the ADAM10 target amyloid precursor protein (24). However, Tspan12 is unlikely to be the only tetraspanin that regulates ADAM10 because it does not share the ubiquitous expression profile of the metalloprotease, instead appearing relatively restricted to endothelial cells of the retina, meninges of the brain, and smooth muscle cells of the colon (16). Indeed, Tspan12 does not appear to be expressed by the platelet (25-29), a cell type on which we chose to initially focus because it expresses many other tetraspanins as well as ADAM10.…”

Secreted and Transmembrane Wnt Inhibitors and Activators

“…TSPAN12 specifically promotes Fz4/LRP5 signaling induced by Norrin, but not by Wnt ligands, to regulate retinal vascular development in mice (Fig. 3C) (Junge et al 2009). …”