Localization of Extracellular Matrix Receptors on the Chondrocyte Primary Cilium

Susan R. McGlashan ^1*, Cynthia G. Jensen ¹ and C. Anthony Poole ¹

¹ Department of Anatomy with Radiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand (SRM,CGJ,CAP), and Section of Orthopaedic Surgery, Department of Medical and Surgical Sciences, Dunedin School of Medicine, University of Otago, Dunedin, New Zealand (CAP)

^* To whom correspondence should be addressed. E-mail: s.mcglashan{at}auckland.ac.nz .

Submitted on October 30, 2005
Accepted on 14 April 2006

	Abstract

A single primary cilium is found in chondrocytes and other connective tissue cells. We have previously shown that extracellular matrix (ECM) macromolecules such as collagen fibers closely associate with chondrocyte primary cilia, and their points of contact are characterized by electron-opaque plaques, suggesting a direct link between the ECM and the cilium. This study examines the expression of receptors for ECM molecules on chondrocyte primary cilia. Embryonic chick sterna were fluorescently labeled with antibodies against and integrins, NG2, CD44 and annexin V. Primary cilia were labeled using acetylated -tubulin antibody. Expression of ECM receptors was examined on chondrocyte plasma membranes and their primary cilia using immunofluorescence and confocal microscopy. All receptors examined showed a punctate distribution on the plasma membrane. 2, 3 and 1-integrins and NG2 were also present on primary cilia, whereas annexin V and CD44 were excluded. The number of receptor-positive cilia varied from 8/50 for NG2 to 43/50 for 1-integrin. This is the first study to demonstrate the expression of integrins and NG2 on chondrocyte primary cilia. The data strongly suggest that chondrocyte primary cilia have the necessary machinery to act as mechanosensors, linking the ECM to cytoplasmic organelles responsible for matrix production and secretion.

Key Words: chondrocytes, primary cilia, integrins, NG2, annexin V, CD44, cartilage, collagen type II, fibronectin

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