Mechanisms linking periodontitis and diabetes

Background + Aims

• Periodontal inflammation stimulates osteoclasts (specialised cells that break down bone) through a signalling protein called receptor activator of nuclear factor kappa-B ligand (RANKL).
• Although immune cells were considered important sources of RANKL, the contribution of osteocytes, which help regulate bone maintenance and remodelling, was unclear, particularly when diabetes worsens periodontitis.
• This study aimed to:
  • Determine whether osteocyte-derived RANKL is necessary for bacteria-induced periodontal bone loss and its aggravation by diabetes.

Materials + Methods

• Researchers compared wild-type mice, with normal RANKL expression, against genetically modified mice in which RANKL production was selectively deleted in the osteocyte lineage.
• Both groups included mice with normal blood glucose and mice with streptozotocin-induced type 1 diabetes, with 5-7 animals per experimental group.
• Diabetes was established approximately 3-weeks before the periodontal experiments began.
• Periodontitis was induced by oral inoculation with Porphyromonas gingivalis and Fusobacterium nucleatum, x3/week for 2-weeks. Tissues were examined 6-weeks after the final inoculation.
• Micro-CT imaging and tissue microscopy assessed alveolar bone area and height. Specialised staining measured osteoclast numbers, eroded bone surfaces and RANKL expression in bone and gingival tissues.

Results

• In wild-type mice, infection reduced the measured bone area between molars by approximately 20% in mice with normal glucose and 42% in diabetic mice, compared with their respective uninfected controls. Micro-CT produced similar findings.
• Deleting osteocyte RANKL prevented significant bone loss. Infected genetically modified mice showed no statistically significant bone loss, whether they had normal glucose or diabetes.
• Bone-resorbing cells increased with infection. Osteoclast numbers rose approximately 4-fold in wild-type mice with normal glucose and sevenfold in diabetic wild-type mice relative to uninfected controls.
• Mice lacking osteocyte-derived RANKL showed no significant infection-induced increase in osteoclast numbers or activity.
• Infection stimulated osteocyte RANKL production in wild-type mice, with the highest expression in the diabetic infected group.
• Gingival RANKL remained present. Infection increased RANKL expression in gingival tissues, with no significant difference between wild-type and genetically modified mice. This supports a particularly important role for osteocyte-derived RANKL in the bone loss observed in this model.

Limitations