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The beneficial role of lubricin in the intervertebral disc is not fully understood
Myron Spector, PhD, answers 4 Questions about how this glycoprotein plays a vital role in the spine.
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Orthopedic surgeons who treat knee and shoulder injuries are reading and hearing more about lubricin and its important function as a lubricant in articular cartilage. It is something we will be hearing more about in the future. Recently, researchers have begun exploring its potential in the intervertebral disc.
I invited Myron Spector, PhD, an Orthopedics Today Editorial Board member, to share his insights and answer this month’s 4 Questions regarding the role of lubricin in musculoskeletal tissues, including the spine.
Background information related to the following interview can be found in the Orthopedics Today 4 Questions interview with Carl R. Flannery, PhD, who addressed lubricin in the context of articular cartilage.
Douglas W. Jackson, MD
Chief Medical Editor
Douglas W. Jackson, MD: What is the definition of lubricin, and how does it function in the human body?
Myron Spector, PhD: Lubricin is a mucinous glycoprotein that can bind to certain tissue surfaces and, by virtue of its structure, can serve as a boundary lubricant. Moreover, lubricin appears to prevent the adhesion of cells and extracellular matrix molecules to the surfaces to which it is coated.
It can be found as a discrete layer on the surface of articular cartilage and the meniscus, and it has been identified as the principal boundary lubricating molecule in joints.
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Jackson: How does it differ from the molecule-superficial zone protein?
Spector: Lubricin, first found in synovial fluid and identified as a product of synovial cells, and superficial zone protein (SZP), a product of the superficial zone chondrocytes of articular cartilage, are encoded by the same gene (PRG4) and are very similar in their molecular structure.
It has been reported that the difference in the molecular weight of lubricin (230–280 kDa) and SZP (345 kDa) is due to the difference in the splicing of exons. Other differences between the two proteins have also been noted.
The similarity in structure between lubricin and SZP suggests that they both serve as lubricating and anti-adhesive proteins. The terms lubricin, SZP and PRG4 are often used interchangeably.
Jackson: Most of us think of lubricin as it relates to articular cartilage. What is or might be the significance of your identifying it in the human intervertebral disc?
Spector: The recent finding of lubricin within the intervertebral disc, and previously within the meniscus and rotator cuff tendons, suggests that it may lubricate the sliding of collagen bundles and fascicles and may play a role in interlamellar tribology.
Its presence at locations which do not clearly appear related to lubrication (eg, in the nucleus pulposus) underscores the fact that we do not yet understand all of the roles that lubricin may be playing in musculoskeletal tissues. Of note is the finding of a discrete layer of lubricin on the surfaces of torn edges of disc samples. Such lubricin layers may serve as barriers to repair processes by preventing integrative binding of the torn edges.
Jackson: What do you predict will be necessary before there are potential clinical applications related to lubricin in joints and/or the spine?
Spector: The finding of a layer of lubricin on the torn edges of tissues — including articular cartilage, tendons and the intervertebral disc — raises the question of whether lubricin may interfere with healing in degenerative and ruptured tissues.
It would be judicious to further investigate the distribution of lubricin and its role as a barrier to healing in these and other pathological conditions before employing injections of exogenous lubricin in the clinic.
While the critically important role of lubricin in the normal function of musculoskeletal tissues continues to emerge, it is also becoming clear that we do not yet understand all of the beneficial roles of this protein, as well as the potential problems related to its distribution in pathological conditions.
For more information:
- Myron Spector, PhD, is professor and director of orthopedic research at Brigham & Women’s Hospital, Harvard Medical School and director of tissue engineering at the VA Boston Health Care System in Boston, and is a member of the Basic Science & Research section on the Orthopedics Today Editorial Board. He can be reached at the VA Boston Health Care System, MS 151, 150 S. Huntington Ave., Boston, MA 02130; 857-364-6639; e-mail: mspector@rics.bwh.harvard.edu. He has no direct financial interest in any companies or products mentioned in this article.