Oral Presentation ANZBMS-MEPSA-ANZORS 2022

Chronic disruption of circadian rhythms (CR) from shiftwork or sleep disorders is associated with bone loss and low bone mineral density (BMD). Glucocorticoid secretion follows a diurnal rhythm and is a potent regulator of CR by synchronising cell-autonomous clocks throughout the body. We therefore investigated whether chronic CR disruption induced-bone loss is mediated through glucocorticoid signalling in osteoblasts/osteocytes.

Mice lacking the glucocorticoid receptor in osteoblasts/osteocytes (obGRKO) and their wild-type (WT) litter-mates were exposed to an established model of chronic CR disruption for 22-weeks. Mice were maintained on either a normal 12hr:12hr light-dark cycle (‘non-shifted’) or exposed to weekly 12-hr phase-shifts, equivalent to spending alternate weeks in Sydney and London (‘shifted’; Fig.1A).

Micro-CT analysis revealed tibiae trabecular BV/TV and BMD were reduced in shifted compared to non-shifted-WT mice. Although non-shifted-obGRKO mice had their own phenotype with lower BV/TV and BMD compared to non-shifted-WT mice, shifted-obGRKO mice had increased BV/TV, while BMD was maintained (Fig.1B-D).

Consistent with lower BMD, shifted-WT mice exhibited a greater proportion of empty osteocyte lacunae, indicative of increased osteocyte apoptosis, compared to non-shifted-WT mice. In contrast, osteocyte apoptosis was similar in shifted and non-shifted obGRKO mice (Fig.1D-F).

To analyse rhythmic changes in serum corticosterone and tibial gene expression, samples were collected at 6am and 6pm. Diurnal rhythmicity of circulating corticosterone was dampened in all shifted mice (Fig.1G) and expression of glucocorticoid target clock genes Bmal1, Per2, and Cry2 were significantly disrupted in tibiae of shifted mice. Notably, rhythmic expression of Bmal1 was flattened in shifted-WT but maintained in shifted-obGRKO mice (Fig.1H). Furthermore, clock target genes including Sost and RankL were increased in shifted-WT mice, whereas Col1a was increased in shifted-obGRKO mice.

In conclusion, local osteoblast/osteocyte glucocorticoid signalling is a critical mediator of bone loss during CR disruption by regulating skeletal clock genes and their downstream rhythmic targets.