Osteocytes regulate organismal senescence of bone and bone marrow

The skeletal system contains a series of sophisticated cellular lineages arisen from the mesenchymal stem cells (MSC) and hematopoietic stem cells (HSC), that determine the homeostasis of bone and bone marrow. Here we reasoned that osteocyte may exert a function in regulation of these lineage cell specifications and tissue homeostasis. Using a mouse model of conditional deletion of osteocytes by the expression of diphtheria toxin subunit (DTA) in dentin matrix protein 1 (DMP-1) positive osteocytes, we demonstrated that partial ablation of DMP-1positive osteocytes caused severe sarcopenia, osteoporosis and degenerative kyphosis, leading to shorter lifespan in these animals. Osteocyte reduction altered mesenchymal lineage commitment resulting in impairment of osteogenesis and induction of osteoclastogensis. Single cell RNA sequencing further revealed that hematopoietic lineage was mobilized towards myeloid lineage differentiation with expanded myeloid progenitors, neutrophils and monocytes, while the lymphopoiesis was impaired with reduced B cells in the osteocyte ablation mice. The acquisition of a senescence-associated secretory phenotype (SASP) in both osteoprogenic and myeloid lineage cells was the underlying cause. Together, we showed that osteocytes play critical roles in regulating of lineage cell specifications in bone and bone marrow through mediation of organismal senescence.