Aging impairs the Treg-related osteogenic induction process via the PGRN/EGFR/PI3K/AKT axis

Aging exerts both direct impacts on osteocytes and indirect influences on bone formation and repair by disrupting the synthesis and secretion of proteins produced by adjacent immune cells. Notably, regulatory T cells (Tregs) have been demonstrated to facilitate the process of osteogenic differentiation. However, the effects of senescent Tregs on osteogenic differentiation and the underlying mechanisms remain to be elucidated.

Experimental investigations performed both in vivo and vitro have effectively induced and characterized the aging of mice Tregs. Conditioned media (CM) were collected from aging and young Tregs populations and subsequently co-cultured with osteoblast precursor cells (MC3T3-E1) to assess alterations in their osteogenic induction capabilities. Proteomic mass spectrometry analysis was then performed to identify critical secreted proteins exhibiting differential expression between aging and young Treg-CM. The osteogenic functions of these proteins were further validated through targeted overexpression and knockdown experiments. Finally, co-immunoprecipitation (Co-IP) and rescue assays were utilized to comprehensively investigate the downstream signaling pathways and underlying molecular mechanisms involved.

In this investigation, we identified an increased proportion of senescent Tregs within the cranial defect region of aging mice, correlating with diminished bone regeneration capacity. Subsequent in vivo and vitro analyses demonstrated that aging Tregs exhibited reduced secretion of progranulin (PGRN), which compromised their ability to induce osteogenesis in murine models. Treatment with recombinant PGRN (rPGRN) effectively reinstated the osteogenic induction potential of aging Tregs. Mechanistic studies revealed that PGRN engages the epidermal growth factor receptor (EGFR) on MC3T3-E1, thereby promoting osteogenic activity through activation of the phosphoinositide 3-kinase/protein kinase B (PI3K/AKT) signaling pathway.

The decreased secretion of PGRN represents a significant factor underlying the impaired osteogenic induction observed in aging Tregs. Our research provides a potential novel therapeutic target and medication strategy to improve bone repair in elderly bone tissue.