No abstract

Advanced macular degeneration is an important cause of vision loss in the United States with over 2 million people affected by the disease. Despite substantial progress in the development of new therapies for wet AMD, the severe visual impairment associated with geographic atrophy in dry AMD or Stargardt disease remains untreatable. Recently, two phase I/II studies involving 18 patients with these diseases have demonstrated that it is possible to safely implant human embryonic stem cell-derived RPE (hESC-RPE) in an attempt to rescue photoreceptors and visual function. The anatomical and functional results are encouraging, with more than half of treated patients experiencing sustained improvements in visual acuity and demonstrating evidence of possible cellular engraftment. However, any conclusions remain tempered by the relatively short follow-up time, lack of a formal control group, poor initial visual acuity, and small number of patients. Aside from an instance of postoperative infectious endophthalmitis, no adverse events related to the cell therapy, such as hyperproliferation, tumorigenicity, or rejection-related inflammation were noted in this initial cohort of 18 patients. These first-in-human safety studies have opened the door to future studies enrolling patients with less advanced disease, treating other diseases that result in RPE loss, employing shorter immunosuppressive regimens, and using alternative strategies for RPE transplantation such as sheets of cells with or without scaffolding to mimic Bruch's membrane. The ultimate goal of these initial safety studies is to promote continued translation of complex biological therapies into meaningful treatment strategies that may address unmet medical needs.

Caveolin-1 is a scaffolding/regulatory protein that interacts with diverse signaling molecules in endothelial cells. To explore the role of this protein in receptormodulated signaling pathways, we transfected bovine aortic endothelial cells (BAEC) with small interfering RNA (siRNA) duplexes to down-regulate caveolin-1 expression. Transfection of BAEC with duplex siRNA targeted against caveolin-1 mRNA selectively "knockeddown" the expression of caveolin-1 by ϳ90%, as demonstrated by immunoblot analyses of BAEC lysates. We used discontinuous sucrose gradients to purify caveolin-containing lipid rafts from siRNA-treated endothelial cells. Despite the near-total down-regulation of caveolin-1 expression, the lipid raft targeting of diverse signaling proteins (including the endothelial isoform of nitric-oxide synthase, Src-family tyrosine kinases, G␣q and the insulin receptor) was unchanged. We explored the consequences of caveolin-1 knockdown on kinase pathways modulated by the agonists sphingosine-1 phosphate (S1P) and vascular endothelial growth factor (VEGF). siRNA-mediated caveolin-1 knockdown enhanced basal as well as S1P-and VEGF-induced phosphorylation of the protein kinase Akt and did not modify the basal or agonist-induced phosphorylation of extracellular signal-regulated kinases 1/2. Caveolin-1 knockdown also significantly enhanced the basal and agonistinduced activity of the small GTPase Rac. We used siRNA to down-regulate Rac expression in BAEC, and we observed that Rac knockdown significantly reduced basal, S1P-, and VEGF-induced Akt phosphorylation, suggesting a role for Rac activation in the caveolin siRNA-mediated increase in Akt phosphorylation. By using siRNA to knockdown caveolin-1 and Rac expression in cultured endothelial cells, we have found that caveolin-1 does not seem to be required for the targeting of signaling molecules to caveolae/lipid rafts and that caveolin-1 differentially modulates specific kinase pathways in endothelial cells.Caveolae are specialized plasmalemmal microdomains that were originally described on the surface of endothelial and epithelial cells (1, 2). First described as endocytic structures, caveolae have been identified as sites for the sequestration of diverse membrane-targeted signaling proteins (for review, see Refs. 3-5). Caveolae are characterized by the presence of the scaffolding/regulatory protein caveolin (6, 7) and by a distinctive lipid composition notable for high concentrations of cholesterol and sphingolipids; the presence of caveolin distinguishes caveolae from other "lipid raft" domains that have a similar lipid composition but do not necessarily contain caveolin.The three caveolin isoforms in mammalian cells are 22-24-kDa integral membrane proteins; caveolin-1 and caveolin-2 are co-expressed in most cell types and are particularly abundant in endothelial cells, whereas caveolin-3 is an isoform that is specific to muscle cells (6). The most extensively characterized member of this protein family, caveolin-1, has been shown to interact with and modulate ...