Dendritic processes of osteocytes are mechanotransducers that induce the opening of hemichannels

Abstract

Osteocytes with long dendritic processes are known to sense mechanical loading, which is essential for bone remodeling. There has been a long-standing debate with regard to which part(s) of osteocyte, the cell body versus the dendritic process, acts as a mechanical sensor. To address this question experimentally, we used a transwell filter system that differentiates the cell body from the dendritic processes. Mechanical loading was applied to either the cell body or the dendrites, and the osteocyte's response was observed through connexin 43 hemichannel opening. The hemichannels located on the cell body were induced to open when mechanical loading was applied to either the dendritic processes or the cell body. However, no significant hemichannel activity in the dendrites was detected when either part of the cell was mechanically stimulated. Disruption of the glycocalyx by hyaluronidase on the dendrite side alone is sufficient to diminish a dendrite's ability to induce the opening of hemichannels on the cell body, while hyaluronidase has no such effect when applied to the cell body. Importantly, hyaluronidase treatment to the dendrite side resulted in formation of poor integrin attachments with the reduced ability of the dendrites to form integrin attachments on the underside of the transwell filter. Together, our study suggests that the glycocalyx of the osteocyte dendritic process is required for forming strong integrin attachments. These integrin attachments probably serve as the mechanotransducers that transmit the mechanical signals to the cell body leading to the opening of hemichannels, which permits rapid exchange of factors important for bone remodeling.

Mechanical loading and how it affects bone cells: The role of the osteocyte cytoskeleton in maintaining our skeleton

“…Direct mechanical perturbation of α5β1 integrins leads to the opening of the Cx43-containing hemi-channels (Batra et al, 2012). Integrin attachments thus likely serve as the mechanotransducing units that potentiate the opening of hemichannels (Burra et al, 2010). Other integrins that may mediate osteocyte mechanotransduction are avb3 integrins.…”

“…The glycocalyx of the osteocyte dendritic process is required for the formation of strong integrin attachments (Burra et al, 2010;Burra et al, 2011), demonstrating that the role of integrins should be considered in the context of the complex structures in which they reside before we can fully comprehend how mechanical stimuli are transduced to osteocytes. It should also be noted that as a consequence of osteocytes expressing their own specific set of integrins and the selectivity of integrins for substrates, the nature of the ECM likely affects the mechanoresponse of osteocytes.…”

Elevated Intracellular Ca2+ Signals by Oxidative Stress Activate Connexin 43 Hemichannels in Osteocytes

“…The depletion of [Ca 2+ ]i by a Ca 2+ chelator or a phospholipase C inhibitor completely prevented the opening of hemichannels induced by OS. To exclude the possibility that BAPTA-AM by itself, but not through the suppression of [Ca 2+ ]i, directly inhibits hemichannel opening, we applied mechanical loading through media dropping to the cell, which has been shown to induce the opening of hemichannels (13). Consistently, media dropping caused an increase of EtBr dye uptake ( Figure 2B).…”

Filamin structure, function and mechanics: are altered filamin-mediated force responses associated with human disease?

“…The final differentiation of bone-forming osteoblast cells to bone remodelling osteocytes is accompanied by alterations to the distribution of FLN, so how FLN's molecular properties facilitates this redistribution would appear crucial for correct bone patterning (Kamioka et al 2004). The cytoskeleton is central to the mechanism by which osteocytes sense fluid shear stress and transduce mechanical signals via an integrin-mediated mechanism to downstream signalling events that impact on bone growth (Burra et al 2010;Wang et al 2007). Mechanisms may involve force-sensitive control of transcription factor activity by retaining regulatory partners in the cytosol; FLN associates with Smads (Sasaki et al 2001;Zheng et al 2007) and PEBP2β (Yoshida et al 2005), proteins that regulate RUNX2 an important transcription factor regulating bone formation (Huang et al 2001).…”