Association between friction and wear in diarthrodial joints lacking lubricin.
OBJECTIVE
The glycoprotein lubricin (encoded by the gene Prg4) is secreted by surface chondrocytes and synovial cells, and has been shown to reduce friction in vitro. In contrast to man-made bearings, mammalian diarthrodial joints must endogenously produce friction-reducing agents. This study was undertaken to investigate whether friction is associated with wear.


METHODS
The lubricating ability of synovial fluid (SF) samples from humans with genetic lubricin deficiency was tested in vitro. The coefficient of friction in the knee joints of normal and lubricin-null mice was measured ex vivo; these joints were also studied by light and electron microscopy. Atomic force microscopy was used to image and measure how lubricin reduces friction in vitro.


RESULTS
SF lacking lubricin failed to reduce friction in the boundary mode. Joints of lubricin-null mice showed early wear and higher friction than joints from their wild-type counterparts. Lubricin self-organized and reduced the work of adhesion between apposing asperities.


CONCLUSIONS
These data show that friction is coupled with wear at the cartilage surface in vivo. They imply that acquired lubricin degradation occurring in inflammatory joint diseases predisposes the cartilage to damage. Lastly, they suggest that lubricin, or similar biomolecules, will have applications in man-made devices in which reducing friction is essential.
Jay
Gregory D
GD
Brown University, Department of Emergency Medicine, Providence, Rhode Island 02903, USA. Gregory_Jay_MD@brown.edu
Torres
Jahn R
JR
Rhee
David K
DK
Helminen
Heikki J
HJ
Hytinnen
Mika M
MM
Cha
Chung-Ja
CJ
Elsaid
Khaled
K
Kim
Kyung-Suk
KS
Cui
Yajun
Y
Warman
Matthew L
ML
D000818:Animals; D002356:Cartilage / Q000473:pathology / Q000502:physiology / Q000648:ultrastructure; D017276:Friction; D006579:Heterozygote; D006720:Homozygote; D006801:Humans; D007592:Joint Diseases / Q000473:pathology / Q000503:physiopathology*; D007719:Knee Joint / Q000473:pathology / Q000503:physiopathology*; D008153:Lubrication; D051379:Mice; D008817:Mice, Mutant Strains; D018625:Microscopy, Atomic Force; D011509:Proteoglycans / Q000235:genetics* / Q000378:metabolism*; D013577:Syndrome; D013582:Synovial Fluid / Q000378:metabolism; D000099158:Proteoglycan 4
D011509:Proteoglycans; C498837:Prg4 protein, mouse; D000099158:Proteoglycan 4