Reduced miR122 Increases Fibroblast Growth Factor 23 (FGF-23) in Mice with CKD

Background: Fibroblast growth factor 23 (FGF23) is associated with mortality in chronic kidney disease (CKD). Iron deficiency and inflammation stimulate the production and subsequent proteolytic cleavage of intact FGF23 (iFGF23) to yield C-terminal FGF23 peptides (Cter-FGF23) that play a protective role in iron metabolism. In CKD, FGF23 cleavage is impaired, but the mechanisms of impaired cleavage are unclear. O-glycosylation of iFGF23 cleavage site by GALNT3 prevents iFGF23 cleavage and we identified micro-RNA 122 (miR122) as an inhibitor of GALNT3. We hypothesized that reduction in osseous miR122 expression contributes to increased levels of GALNT3 and iFGF23 in CKD mice. Methods: We crossed mice harboring a conditional deletion of miR122 in osteoblasts (miR122Osx-cKO) with Col4a3KO mice with CKD to generate WT, miR122Osx-cKO, Col4a3KO, and compound (CPD) Col4a3KO/miR122Osx-cKO mice. In all mice, we analyzed serum biochemical, hematological parameters and bone and liver gene expression at 18-weeks of age. Results: Compared to WT mice, miR122Osx-cKO mice showed reduced bone miR122 levels, increased bone Galnt3 expression, and reduced iron levels and transferrin saturation, but normal hemoglobin levels and red blood cells counts. As previously shown, Col4a3KO mice with advanced CKD displayed reduced iron levels and transferrin saturation and were overtly anemic compared to WT mice. They also showed a 2-fold reduction in bone miR122 levels, increased bone Galnt3 expression (2.2-fold), and a dramatic increase in iFGF23 (7-fold). Further reduction in miR122 in CPD mice led to a 30% increase in bone Galnt3 expression, a consequent 50% rise in iFGF23 circulating levels, and 20% increase in i/cFGF23 ratio vs. Col4a3KO mice, suggesting a further reduction of FGF23 proteolytic cleavage. As a consequence of reduced iron conserving Cter-FGF23 peptides, CPD mice also showed further reductions in hemoglobin and red blood cell. Conclusion: Our study demonstrates that reduced expression of osseous miR122 in CKD leads to impaired FGF23 processing, resulting in reduced Cter-FGF23 but increased iFGF23. Therefore, administration of miR122 mimics to mice with CKD could lower iFGF23 and improve CKD-associated adverse outcomes. Funding: NIDDK Support