Loss of <i>Nmp4</i> optimizes osteogenic metabolism and secretion to enhance bone quality

Abstract

A goal of osteoporosis therapy is to restore lost bone with structurally sound tissue. Mice lacking the transcription factor nuclear matrix protein 4 (<i>Nmp4</i>, <i>Zfp384</i>, <i>Ciz</i>, <i>ZNF384</i>) respond to several classes of osteoporosis drugs with enhanced bone formation compared with wild-type (WT) animals. <i>Nmp4^-/-</i> mesenchymal stem/progenitor cells (MSPCs) exhibit an accelerated and enhanced mineralization during osteoblast differentiation. To address the mechanisms underlying this hyperanabolic phenotype, we carried out RNA-sequencing and molecular and cellular analyses of WT and <i>Nmp4^-/-</i> MSPCs during osteogenesis to define pathways and mechanisms associated with elevated matrix production. We determined that <i>Nmp4</i> has a broad impact on the transcriptome during osteogenic differentiation, contributing to the expression of over 5,000 genes. Phenotypic anchoring of transcriptional data was performed for the hypothesis-testing arm through analysis of cell metabolism, protein synthesis and secretion, and bone material properties. Mechanistic studies confirmed that <i>Nmp4^-/-</i> MSPCs exhibited an enhanced capacity for glycolytic conversion: a key step in bone anabolism. <i>Nmp4^-/-</i> cells showed elevated collagen translation and secretion. The expression of matrix genes that contribute to bone material-level mechanical properties was elevated in <i>Nmp4^-/-</i> cells, an observation that was supported by biomechanical testing of bone samples from <i>Nmp4^-/-</i> and WT mice. We conclude that loss of <i>Nmp4</i> increases the magnitude of glycolysis upon the metabolic switch, which fuels the conversion of the osteoblast into a super-secretor of matrix resulting in more bone with improvements in intrinsic quality.