Effect of Ca2+ Ions on the Adhesion and Mechanical Properties of Adsorbed Layers of Human Osteopontin

Using an atomic force microscope and a surface force apparatus, we measured the surface coverage, adhesion, and mechanical properties of layers of osteopontin (OPN), a phosphoprotein of the human bones, adsorbed on mica. OPN is believed to connect mineralized collagen fibrils of the bone in a matrix...

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Bibliographic Details
Published inBiophysical journal Vol. 95; no. 6; pp. 2939 - 2950
Main Authors Zappone, Bruno, Thurner, Philipp J., Adams, Jonathan, Fantner, Georg E., Hansma, Paul K.
Format Journal Article
LanguageEnglish
Published United States Elsevier Inc 15.09.2008
The Biophysical Society
Subjects
Adsorption - drug effects
Aluminum Silicates - metabolism
Biomechanical Phenomena
Buffers
Calcium - pharmacology
Humans
Microscopy, Atomic Force
Osteopontin - chemistry
Osteopontin - metabolism
Proteins
Sodium Hydroxide - chemistry
Surface Properties
Time Factors
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Summary:Using an atomic force microscope and a surface force apparatus, we measured the surface coverage, adhesion, and mechanical properties of layers of osteopontin (OPN), a phosphoprotein of the human bones, adsorbed on mica. OPN is believed to connect mineralized collagen fibrils of the bone in a matrix that dissipates energy, reducing the risk of fractures. Atomic force microscopy normal force measurements showed large adhesion and energy dissipation upon retraction of the tip, which were due to the breaking of the many OPN-OPN and OPN-mica bonds formed during tip-sample contact. The dissipated energy increased in the presence of Ca2+ ions due to the formation of additional OPN-OPN and OPN-mica salt bridges between negative charges. The forces measured by surface force apparatus between two macroscopic mica surfaces were mainly repulsive and became hysteretic only in the presence of Ca2+: adsorbed layers underwent an irreversible compaction during compression due to the formation of long-lived calcium salt bridges. This provides an energy storage mechanism, which is complementary to energy dissipation and may be equally relevant to bone recovery after yield. The prevalence of one mechanism or the other appears to depend on the confinement geometry, adsorption protocol, and loading-unloading rates.
Bibliography:Address reprint requests to Bruno Zappone, E-mail: zappone@fis.unical.it.
Editor: Jane Clarke.
Bruno Zappone and Philipp J. Thurner contributed equally to this work.
ISSN:0006-3495
1542-0086
DOI:10.1529/biophysj.108.135889