# Inflammatory cytokines secreted from senescent periodontal ligament cells influence the osteocyte network in alveolar bone

## TL;DR

Aging reduces alveolar bone remodeling by impairing osteocyte networks, likely due to inflammatory signals from aged periodontal cells.

## Contribution

Identifies senescence-associated secretory phenotype (SASP) factors from periodontal ligament cells as a novel contributor to age-related alveolar bone deterioration.

## Key findings

- Senescent periodontal ligament cells secrete SASP factors that impair osteocyte network formation in alveolar bone.
- Senolytic treatment with Dasatinib and Quercetin preserves osteocyte canalicular networks in aged mice.
- SASP inhibition increases E11 expression and reduces sclerostin in aged alveolar bone.

## Abstract

The rate of orthodontic tooth movement slows with age, likely due to changes in alveolar bone remodeling. However, the underlying mechanisms remain poorly understood. Osteocytes (OCys), which form the lacunar-canalicular network (LCN), play a key role in bone metabolism; however, the effects of aging on the alveolar LCN are still unclear. This study aimed to clarify age-related alterations in the LCN of alveolar bone and their underlying mechanisms. The LCN was visualized using Ploton silver staining, revealing a significant reduction in 16-mo-old C57BL/6J mice compared to 2-mo-old mice. The number of OCys, as well as the number and length of canaliculi per OCy, decreased significantly with age. These reductions were consistent in both sexes. Periodontal ligament cells (PDLCs) from aged mice showed increased expression of the senescence marker p16INK4A and upregulation of senescence-associated secretory phenotype (SASP) factor mRNAs. To evaluate the effect of SASP factors on LCN formation, a 3D-block culture system was utilized. Conditioned medium (CM) from senescent human PDLCs (over 20 passages) suppressed dendrite formation in the 3D culture as well as the expression of E11/podoplanin, while CM from young PDLCs had no significant effect. To test whether SASP inhibition could preserve LCN, the senolytic agents Dasatinib and Quercetin (D + Q) were administered to 12-mo-old mice for 4 mo. D + Q treatment significantly increased both canalicular number and length per OCy in alveolar bone compared to vehicle controls. In addition, D + Q treatment increased E11 expression and suppressed sclerostin expression on OCys in aged alveolar bones. These findings suggest that SASP factors from senescent PDLCs contribute to the age-related LCN deterioration in alveolar bone, potentially impairing OCy function and bone remodeling. Targeting PDL cellular senescence may therefore offer a novel strategy for preserving alveolar bone integrity with aging.

Graphical Abstract