Agrin-deficient osteocytes disrupt bone tissue homeostasis in male mice

Osteocytes are terminally differentiated osteoblasts that secrete molecules that regulate bone-tissue homeostasis. Considering that the extracellular matrix protein agrin (AGRN) is secreted by osteoblasts and modulates their differentiation, we hypothesized that AGRN is also expressed by osteocytes and plays a role in their function and therefore in bone remodeling. To test this hypothesis, we deleted agrin specifically in osteocytes using dentin matrix acidic phosphoprotein 1 (DMP1)-Cre mice (C57/BL6 background) and silenced agrin in vitro using clustered regularly interspaced short palindromic repeats/associated nuclease Cas-9 in the Ocy454 osteocyte cell line. We found that osteocytes express agrin and its receptors, low-density lipoprotein receptor-related protein 4, and α-dystroglycan, and that mice with agrin-deficient osteocytes exhibited lower bone mass and impaired mechanical and chemical properties of bone tissue. Agrin knockdown in Ocy454 cells disrupted osteocyte differentiation and function, which reduced osteoblast and increased osteoclast differentiation in a cell co-culture model. Our results showed that agrin is expressed by osteocytes, which are key regulators of bone mass and its mechanical and chemical properties. These findings indicate that agrin may be a therapeutic target because it is important to maintain the balance of the osteocyte-osteoblast-osteoclast circuit, and consequently, bone tissue homeostasis.