Osteocytes and Primary Cilia

Abstract

Purpose of review: The purpose of this review is to provide a background on osteocytes and the primary cilium, discussing the role it plays in osteocyte mechanosensing.

Recent findings: Osteocytes are thought to be the primary mechanosensing cells in bone tissue, regulating bone adaptation in response to exercise, with the primary cilium suggested to be a key mechanosensing mechanism in bone. More recent work has suggested that, rather than being direct mechanosensors themselves, primary cilia in bone may instead form a key chemo-signalling nexus for processing mechanoregulated signalling pathways. Recent evidence suggests that pharmacologically induced lengthening of the primary cilium in osteocytes may potentiate greater mechanotransduction, rather than greater mechanosensing. While more research is required to delineate the specific osteocyte mechanobiological molecular mechanisms governed by the primary cilium, it is clear from the literature that the primary cilium has significant potential as a therapeutic target to treat mechanoregulated bone diseases, such as osteoporosis.

Keywords: Biomechanics; Bone; Mechanobiology; Osteocyte; Primary cilium.

Fig. 1

Osteocytes sense mechanical stimulation in vivo, with a number of potential sensing mechanisms identified (black and white arrows indicate mechanosensors): (A) TEM image of an osteocyte process displaying the actin cytoskeleton; (B) TEM image of proteoglycan pericellular matrix (PCM) tethering elements (black arrows) bridging an osteocyte cell process to the bony canalicular wall; (C) fluorescent immunohistochemical staining showing that β1 integrins (white arrows) are located only on osteocyte cell bodies; (D) TEM image demonstrating the discrete extracellular matrix (ECM) projections from the canalicular wall that contact osteocyte processes (black arrows); (E) fluorescent immunohistochemical staining for β3 integrins (white arrows) that are present in a punctate pattern along osteocyte processes, with similar periodicity and spacing pattern to ECM projections; and (F) the primary cilium (white arrow) on the osteocyte cell body. These sensing mechanisms may experience either interstitial fluid flow or strain of the surrounding matrix: (G) combined fluid shear and matrix strain via tethering elements or focal adhesions along the dendritic cell processes; (H) matrix strain via focal adhesions on the cell body; and (I) direct fluid flow sensing via the primary cilium in the lacunar cavity. Adapted from Schaffler et al. [1], Verbruggen and McNamara [21], and Duffy and Jacobs [22]

Fig. 2

(A) Configuration of the osteocyte primary cilium when cultured in vitro, or when in situ in its lacuna in vivo. Adapted from Qin et al. [36]. (B) Computational models predict that high strain is likely experienced by both integrin attachments and the primary cilium under in vitro flow conditions. However, while integrin attachments were similarly stimulated under in vivo flow, the primary cilium was only sufficiently mechanically stimulated if connected to the surrounding matrix. Adapted from Vaughan et al. [37]. (C) Evidence of hyaluronic acid (red), a key component of the cell glycocalyx and the pericellular matrix, present on the entire cell surface of MLO-A5 osteoblastic cells in vitro, an co-localising along the length of the primary cilium (acetylated α-tubulin, green) [38]. Mechanical loading upregulates collagen type I and osteopontin by MLO-A5 osteoblastic cells, with this effect blocked by pre-treatment with chloral-hydrate to disrupt primary cilia, as measured by relative gene expression via RT-PCR, shown as (D) gels and (E) fold-change compared to their controls as measured by band density relative to GAPDH. (*p < 0.05, Tukey’s post hoc pairwise comparison, n = 3)