Mir-9-1 controls osteoblastic regulation of lymphopoiesis

The highly conserved MicroRNA-9 (miR-9) family consists of three members. We discovered that miR-9-1 deletion reduced mature miR-9 expression, causing 43% of the mice to display smaller size and postweaning lethality. MiR-9-1-deficient mice with growth defects experienced severe lymphopenia, but other blood cells were unaffected. The lymphopenia wasn't due to defects in hematopoietic progenitors, as mutant bone marrow (BM) cells underwent normal lymphopoiesis after transplantation into wild-type recipients. Additionally, miR-9-1-deficient mice exhibited impaired osteoblastic bone formation, as mutant mesenchymal stem cells (MSCs) failed to differentiate into osteoblastic cells (OBs). RNA sequencing revealed reduced expression of master transcription factors for osteoblastic differentiation, Runt-related transcription factor 2 (Runx2) and Osterix (Osx), and genes related to collagen formation, extracellular matrix organization, and cell adhesion, in miR-9-1-deficient MSCs. Follistatin (Fst), an antagonist of bone morphogenetic proteins (BMPs), was found to be a direct target of miR-9-1. Its deficiency led to the up-regulation of Fst, inhibiting BMP signaling in MSCs, and reducing IL-7 and IGF-1. Thus, miR-9-1 controls osteoblastic regulation of lymphopoiesis by targeting the Fst/BMP/Smad signaling axis. Overall design: To explore the potential regulation of miR-9-1 in the lymphopoietic BM niche, we used CRISPR/Cas9-meidated genome editing to generate miR-9-1-deficient mice. We then performed a high-throughput RNA sequencing analysis of global gene expression patterns in MSCs and OBs from 4 miR-9-1-deficient and wild-type mice. Comparative global gene expression patterns analysis of RNA-seq data for miR-9-1-deficietn and wild-type MSCs and OBs.