The trafficking of circulating stem and progenitor cells to areas of tissue damage is poorly understood. The chemokine stromal cell-derived factor-1 (SDF-1 or CXCL12) mediates homing of stem cells to bone marrow by binding to CXCR4 on circulating cells. SDF-1 and CXCR4 are expressed in complementary patterns during embryonic organogenesis and guide primordial stem cells to sites of rapid vascular expansion. However, the regulation of SDF-1 and its physiological role in peripheral tissue repair remain incompletely understood. Here we show that SDF-1 gene expression is regulated by the transcription factor hypoxia-inducible factor-1 (HIF-1) in endothelial cells, resulting in selective in vivo expression of SDF-1 in ischemic tissue in direct proportion to reduced oxygen tension. HIF-1-induced SDF-1 expression increases the adhesion, migration and homing of circulating CXCR4-positive progenitor cells to ischemic tissue. Blockade of SDF-1 in ischemic tissue or CXCR4 on circulating cells prevents progenitor cell recruitment to sites of injury. Discrete regions of hypoxia in the bone marrow compartment also show increased SDF-1 expression and progenitor cell tropism. These data show that the recruitment of CXCR4-positive progenitor cells to regenerating tissues is mediated by hypoxic gradients via HIF-1-induced expression of SDF-1.

Clinical trials of small interfering RNA (siRNA) targeting vascular endothelial growth factor-A (VEGFA) or its receptor VEGFR1 (also called FLT1), in patients with blinding choroidal neovascularization (CNV) from age-related macular degeneration, are premised on gene silencing by means of intracellular RNA interference (RNAi). We show instead that CNV inhibition is a siRNA-class effect: 21-nucleotide or longer siRNAs targeting non-mammalian genes, non-expressed genes, non-genomic sequences, pro-and anti-angiogenic genes, and RNAi-incompetent siRNAs all suppressed CNV in mice comparably to siRNAs targeting Vegfa or Vegfr1 without off-target RNAi or interferon-a/b activation. Non-targeted (against non-mammalian genes) and targeted (against Vegfa or Vegfr1) siRNA suppressed CNV via cell-surface toll-like receptor 3 (TLR3), its adaptor TRIF, and induction of interferon-c and interleukin-12. Non-targeted siRNA suppressed dermal neovascularization in mice as effectively as Vegfa siRNA. siRNA-induced inhibition of neovascularization required a minimum length of 21 nucleotides, a bridging necessity in a modelled 2:1 TLR3-RNA complex. Choroidal endothelial cells from people expressing the TLR3 coding variant 412FF were refractory to extracellular siRNA-induced cytotoxicity, facilitating individualized pharmacogenetic therapy. Multiple human endothelial cell types expressed surface TLR3, indicating that generic siRNAs might treat angiogenic disorders that affect 8% of the world's population, and that siRNAs might induce unanticipated vascular or immune effects.Therapeutic application of long, double-stranded (ds)RNAmediated RNAi and sequence-specific gene silencing through RNAi by short synthetic RNA duplexes is challenging because mammalian cells do not uptake 'naked' siRNA (whether chemically modified or not) without cell-permeating entities [1][2][3][4] . To minimize systemic exposure, initial clinical trials of siRNA were launched using intraocular injection in patients with CNV. CNV, wherein the retina is invaded by choroidal vessels beneath the retinal pigmented epithelium (RPE), is a late stage of age-related macular degeneration that afflicts 30-50 million people globally. The preclinical bases for trials of naked VEGFA siRNA (Bevasiranib) or VEGFR1 siRNA (AGN211745/ siRNA-027) were single reports in mice 5,6 that such siRNAs suppressed laser-injury-induced CNV, a model predictive of efficacy in humans 7,8 . These findings were interpreted as anomalous examples of local delivery surmounting the impediment to intracellular entry 9-11 . Instead, we show in two animal models that suppression of neovascularization is a generic property of siRNAs independent of sequence, target and internalization.Sequence-independent angiogenesis suppression by siRNA Numerous synthetic non-targeted 21-nucleotide duplex siRNAs from multiple vendors, when injected into the vitreous humour of wild-type mice, uniformly and dose-dependently suppressed CNV (Fig. 1a, b and Supplementary Fig. 1). siRNAs targeting jellyfish green fluorescent ...

SUMMARY Alu RNA accumulation due to DICER1 deficiency in the retinal pigmented epithelium (RPE) is implicated in geographic atrophy (GA), an advanced form of age-related macular degeneration that causes blindness in millions of individuals. The mechanism of Alu RNA-induced cytotoxicity is unknown. Here we show that DICER1 deficit or Alu RNA exposure activates the NLRP3 inflammasome and triggers TLR-independent MyD88 signaling via IL-18 in the RPE. Genetic or pharmacological inhibition of inflammasome components (NLRP3, Pycard, Caspase-1), MyD88, or IL-18 prevents RPE degeneration induced by DICER1 loss or Alu RNA exposure. These findings, coupled with our observation that human GA RPE contains elevated amounts of NLRP3, PYCARD and IL-18, and evidence of increased Caspase-1 and MyD88 activation, provide a rationale for targeting this pathway in GA. Our findings also reveal a function of the inflammasome outside the immune system and an immunomodulatory action of mobile elements.

Geographic atrophy (GA), an untreatable advanced form of age-related macular degeneration, results from retinal pigmented epithelium (RPE) cell death. Here we show that the microRNA (miRNA)-processing enzyme DICER1 is reduced in the RPE of humans with GA, and that conditional ablation of Dicer1, but not seven other miRNA-processing enzymes, induces RPE degeneration in mice. DICER1 knockdown induces accumulation of Alu RNA in human RPE cells and Alu-like B1 and B2 RNAs in mouse RPE. Alu RNA is increased in the RPE of humans with GA, and this pathogenic RNA induces human RPE cytotoxicity and RPE degeneration in mice. Antisense oligonucleotides targeting Alu/B1/B2 RNAs prevent DICER1 depletion-induced RPE degeneration despite global miRNA downregulation. DICER1 degrades Alu RNA, and this digested Alu RNA cannot induce RPE degeneration in mice. These findings reveal a miRNA-independent cell survival function for DICER1 involving retrotransposon transcript degradation, show that Alu RNA can directly cause human pathology, and identify new targets for a major cause of blindness.