Targeting Calcium‐sensing Receptor May Help Slow Disease Progression

Targeting the calcium‐sensing receptor (CaSR), which plays a critical role in the development of bone cells, may be a therapeutic strategy to slow the progression of ankylosing spondylitis (AS), a study suggests.

The study, “Aberrant upregulation of CaSR promotes pathological new bone formation in ankylosing spondylitis,” was published in the journal EMBO Molecular Medicine.

AS is a form of arthritis that primarily affects the joints of the spine. Patients may develop ankylosis, which results from new bone formation and involves fusion of sections of the spine.

Recent studies and treatments have focused on controlling the symptoms of AS, but the mechanisms underlying new bone formation and inflammation are still unclear.

CaSR, which is highly involved in many inflammatory diseases, has a critical role in modulating calcium levels, which might affect bone mass. However, its role in AS remains unknown. As such, researchers at the Sun Yat‐sen University, in China, led an investigation of CaSR levels in AS patients and in animal models. The study included 42 patients (18 with AS) admitted to two hospitals between September 2013 and June 2019.

Cells containing CaSR accumulated in spinal tissues from AS patients who underwent correction surgery, compared with age- and sex-matched controls. The majority of these cells expressed OCN, a marker of mature osteoblasts (bone-forming cells) and accumulated in the tissue connecting ligament and bone.

The team then performed studies in mouse models. In proteoglycan-induced spondylitis mice, spinal ankylosis gradually developed to 73.3% of animals at 24 weeks (nearly six months). Expression of CaSR (RNA production, an intermediate to proteins) was increased in spinal ligament tissues, and new bone formation was observed throughout this period.

In turn, the DBA/1 arthritis model, which shares similarities with AS in humans, further showed increased expression of CaSR and new bone formation in the hind paws over 20 weeks.

In another model used to study mechanical stress in AS, bony projections gradually enlarged while the number of fibrocartilage cells containing CaSR increased over eight weeks.

In a subsequent experiment, the team treated mice with NPS-2143, a CaSR blocker. Systemic treatment reduced new bone formation and reduced the severity of ankylosis compared to a control group.

Administration of a CaSR activator called strontium ranelate (SR) failed to promote the formation of cartilage cells (chondrogenesis). In turn, NPS‐2143 suppressed this process, as well as the activation of chondrogenic marker genes, Sox9 and Col2a1, suggesting that its effect in ankylosis might involve the inhibition of cartilage replacement by bone, said the investigators.

In human bone marrow stem cells — able to continuously divide and differentiate into other cell types — calcium deposition was increased after administration of SR and decreased when NPS‐2143 was used. Similar effects were observed in marker genes, such as Runx2, Osx, ALP, and OCN, suggesting that CaSR might be critical for the differentiation of bone-forming cells (osteoblasts), the team added.

The data also showed that CaSR promoted bone formation through the activation of the phospholipase c gamma (PLCγ) biological pathway. Multiple inflammatory cytokines — small proteins secreted by cells and important in cell signaling — including TNF-alpha, promoted the activation (upregulation) of CaSR in bone-forming cells.

“These novel findings suggest that inflammation‐induced aberrant upregulation of CaSR and activation of CaSR‐PLCγ signalling in osteoblasts act as mediators of inflammation, affecting pathological new bone formation in AS,” the researchers wrote.

The study’s limitations included having no data on the role of CaSR in other cell types and the absence of an appropriate technique to detect calcium outside cells.

“Our findings revealed a prominent role for CaSR in the interplay between inflammation and the process of ankylosis progression, which might shed more light on the enigma of inflammation‐related pathological new bone formation in AS and propose a potential therapeutic target for slowing ankylosis progression,” the researchers concluded.