Mechanisms by which Small Nucleolar RNAs Exacerbate Atherosclerosis

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Superoxide and other derivative reactive oxygen species (ROS) promote atherosclerosis (athero) as well as vascular smooth muscle cell (SMC) and macrophage inflammatory signaling. Anti-atherogenic strategies targeting O2--producing NADPH oxidases, however, increase susceptibility to infection. This project’s goal is to discern novel mechanisms for constraining ROS-promoted atherogenesis while minimizing adverse effects on immunity. One such mechanism may involve the ubiquitously expressed noncoding small nucleolar (sno) RNAs from the ribosomal protein L13a (Rpl13a) locus: SNORD32A, SNORD33, SNORD34, and SNORD35A. We found that these snoRNAs augment ROS levels and oxidative stress in vitro and in vivo. Our Preliminary Studies with Rpl13a-snoRNA-/- (snoKO) mice and SMCs derived from them show: (1) snoKO SMCs have lower levels of ROS, cell proliferation and migration than congenic WT SMCs. (2) Compared with WT SMCs, snoKO SMCs express 5.7-fold more cytochrome C oxidase subunit 4 isoform 2 (COX4I2), which reduces mitochondrial O2- production. (3) SnoKO carotid arteries develop less athero than WT carotids when transplanted orthotopically into Apoe-/- mice. (4) Compared with Apoe-/- mice, snoKO/Apoe-/- mice develop 40% less brachiocephalic athero. (5) Compared with snoRNA+/+ brachiocephalic arteries or carotid grafts, snoKO arteries demonstrate less SMC-to-foam-cell transdifferentiation, a process potentiated by ROS. SnoRNAs bind to their target RNAs via an antisense domain, then recruit the enzyme fibrillarin, which effects RNA 2’-O-methylation. SnoRNAs canonically modify ribosomal RNA; however, we discovered that at least one of the Rpl13a snoRNAs can target mRNA for 2’-O-methylation—a process that alters mRNA abundance and translation. Nonetheless, specific mRNAs that constitute targets for pro-oxidant effects of Rpl13a-snoRNAs remain obscure. This project will therefore test the hypotheses that Rpl13a snoRNAs promote athero, particularly by potentiating SMC-to-foam cell transdifferentiation, and that that Rpl13a-snoRNA-guided mRNA 2’-O-methylation affects protein expression of key ROS-regulating enzyme(s) in SMCs and Mφs, including COX4I2. To do so, this project will compare athero in Rpl13a-snoRNA-/-/Apoe-/- versus Apoe-/- mice, and use bone marrow transplantation to discern the roles of Rpl13a-snoRNAs in bone marrow-derived cells versus arterial wall-derived cells. We will investigate how Rpl13a-snoRNAs affect foam cell formation in macrophages and SMCs, and determine whether COX4I2 engenders lower ROS levels and inflammation in snoKO SMCs. Finally, we will identify mRNA targets of Rpl13a-snoRNAs in SMCs and macrophages, by performing transcriptome-wide mapping of 2’-O-methylation sites on mRNA from WT and Rpl13a-snoRNA-/- SMCs and macrophages, by using the RibOxi-seq and crosslinking, ligation, and sequencing of hybrids (CLASH) approach. By elucidating mechanisms by which snoRNAs regulate ROS in SMCs and macrophages, this project should identify new therapeutic targets for athero.