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Molecular and Cellular Biology, June 2006, p. 4539-4552, Vol. 26, No. 12
0270-7306/06/$08.00+0     doi:10.1128/MCB.02120-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

E11/gp38 Selective Expression in Osteocytes: Regulation by Mechanical Strain and Role in Dendrite Elongation

Keqin Zhang ,{dagger},1 Cielo Barragan-Adjemian,1 Ling Ye,1 Shiva Kotha,1 Mark Dallas,1 Yongbo Lu,1 Shujie Zhao,2 Marie Harris,3 Stephen E. Harris,3 Jian Q. Feng,1 and Lynda F. Bonewald1*

Department of Oral Biology, School of Dentistry, University of Missouri, Kansas City, Missouri 64108,1 Departments of Medicine,2 Periodontics, University of Texas Health Science Center, San Antonio, Texas 78229-39003

Received 1 November 2005/ Returned for modification 1 December 2005/ Accepted 3 April 2006

Within mineralized bone, osteocytes form dendritic processes that travel through canaliculi to make contact with other osteocytes and cells on the bone surface. This three-dimensional syncytium is thought to be necessary to maintain viability, cell-to-cell communication, and mechanosensation. E11/gp38 is the earliest osteocyte-selective protein to be expressed as the osteoblast differentiates into an osteoid cell or osteocyte, first appearing on the forming dendritic processes of these cells. Bone extracts contain large amounts of E11, but immunostaining only shows its presence in early osteocytes compared to more deeply embedded cells, suggesting epitope masking by mineral. Freshly isolated primary osteoblasts are negative for E11 expression but begin to express this protein in culture, and expression increases with time, suggesting differentiation into the osteocyte phenotype. Osteoblast-like cell lines 2T3 and Oct-1 also show increased expression of E11 with differentiation and mineralization. E11 is highly expressed in MLO-Y4 osteocyte-like cells compared to osteoblast cell lines and primary osteoblasts. Differentiated, mineralized 2T3 cells and MLO-Y4 cells subjected to fluid flow shear stress show an increase in mRNA for E11. MLO-Y4 cells show an increase in dendricity and elongation of dendrites in response to shear stress that is blocked by small interfering RNA specific to E11. In vivo, E11 expression is also increased by a mechanical load, not only in osteocytes near the bone surface but also in osteocytes more deeply embedded in bone. Maximal expression is observed not in regions of maximal strain but in a region of potential bone remodeling, suggesting that dendrite elongation may be occurring during this process. These data suggest that osteocytes may be able to extend their cellular processes after embedment in mineralized matrix and have implications for osteocytic modification of their microenvironment.

* Corresponding author. Mailing address: Department of Oral Biology, University of Missouri at Kansas City, 630 East 25th Street, Kansas City, MO 64108-2784. Phone: (816) 235-2068. Fax: (816) 235-5524. E-mail: bonewaldl{at}umkc.edu.

{dagger} Present address: The First Affiliated Hospital, Nanjing Medical University, Nanjing 210029, People’s Republic of China.

Molecular and Cellular Biology, June 2006, p. 4539-4552, Vol. 26, No. 12
0270-7306/06/$08.00+0     doi:10.1128/MCB.02120-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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