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Cilia. 2015 May 29;4:7. doi: 10.1186/s13630-015-0016-y. eCollection 2015.

The primary cilium functions as a mechanical and calcium signaling nexus.

Author information

1
Department of Biomedical Engineering, Columbia University, 351 Engineering Terrace, MC 8904, 1210 Amsterdam Ave, New York, NY 10027 USA.
2
Department of Biomedical Engineering, Columbia University, 351 Engineering Terrace, MC 8904, 1210 Amsterdam Ave, New York, NY 10027 USA ; Jacobs Technion-Cornell Innovation Institute, Cornell Tech, New York, NY 10011 USA.
3
Department of Biomedical Engineering, Columbia University, 351 Engineering Terrace, MC 8904, 1210 Amsterdam Ave, New York, NY 10027 USA ; Department of Biotechnology, University of British Columbia, Vancouver, BC V6T 1Z4 Canada.
4
Bioengineering Department, UC San Diego, La Jolla, CA 92093 USA.

Abstract

BACKGROUND:

The primary cilium is an antenna-like, nonmotile structure that extends from the surface of most mammalian cell types and is critical for chemosensing and mechanosensing in a variety of tissues including cartilage, bone, and kidney. Flow-induced intracellular calcium ion (Ca(2+)) increases in kidney epithelia depend on primary cilia and primary cilium-localized Ca(2+)-permeable channels polycystin-2 (PC2) and transient receptor potential vanilloid 4 (TRPV4). While primary cilia have been implicated in osteocyte mechanotransduction, the molecular mechanism that mediates this process is not fully understood. We directed a fluorescence resonance energy transfer (FRET)-based Ca(2+) biosensor to the cilium by fusing the biosensor sequence to the sequence of the primary cilium-specific protein Arl13b. Using this tool, we investigated the role of several Ca(2+)-permeable channels that may mediate flow-induced Ca(2+) entry: PC2, TRPV4, and PIEZO1.

RESULTS:

Here, we report the first measurements of Ca(2+) signaling within osteocyte primary cilia using a FRET-based biosensor fused to ARL13B. We show that fluid flow induces Ca(2+) increases in osteocyte primary cilia which depend on both intracellular Ca(2+) release and extracellular Ca(2+) entry. Using siRNA-mediated knockdowns, we demonstrate that TRPV4, but not PC2 or PIEZO1, mediates flow-induced ciliary Ca(2+) increases and loading-induced Cox-2 mRNA increases, an osteogenic response.

CONCLUSIONS:

In this study, we show that the primary cilium forms a Ca(2+) microdomain dependent on Ca(2+) entry through TRPV4. These results demonstrate that the mechanism of mechanotransduction mediated by primary cilia varies in different tissue contexts. Additionally, we anticipate that this work is a starting point for more studies investigating the role of TRPV4 in mechanotransduction.

KEYWORDS:

Biosensor; Calcium signaling; Kidney epithelia; Mechanotransduction; Osteocyte; Primary cilium

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