Osterix Facilitates Osteocytic Communication by Targeting Connexin43

Abstract

Osteocytes, terminal-differentiated cells in bone, are now considered as more pivotal regulators of mature bone homeostasis than other bone cells, since they constitute 90-95% of the bone cell population. Given their non-migratory nature within the mineralized matrix, their unique dendrites are crucial for cell-to-cell communication in response to both intracellular and extracellular stimuli, such as bone fracture or mechanical load. Here, we showed that Osterix (Osx), usually recognized as a specific doorkeeper for osteoblast differentiation during new bone formation marked by collagen type I α 1 (Col1α1), was unexpectedly co-expressed with Col1α1 in osteocytes within the cortical bone of mice. Deleting Osx in Col1α1-positive osteocytes disrupted cortical bone structure and osteocytic dendrites in mice, thus impairing transcellular fluid flow and intercellular communication. Conversely, overexpression of Osx in osteocytes enhanced these processes. Furthermore, we identified Connexin43, a critical protein of gap junction channel, was a direct transcriptional target of Osx in regulating dendrites of osteocytes. Pharmacological restoration of Connexin43 levels rescued the dysfunction in Osx-deficient osteocytes both in vitro and in vivo. Taken together, this work demonstrated Osx's distinct role in osteocyte function through maintaining intercellular signaling, which broadened the current understanding of its role in Col1α1-positive bone cells, extending beyond osteoblasts and bone mineralization, offering new insights into bone diseases such as fracture nonunion or disuse osteoporosis.