Lack Of Specific Collagen Type Leads To Osteoarthritis
Duke University Medical Center researchers have found
that joints whose cartilage lacks a specific type of collagen will
develop osteoarthritis – the so-called "wear-and-tear"
form of the disease – at a greatly accelerated rate.
The results of their experiments with mice provide
new insights that could lead to potential treatments for a disease
that afflicts more than 40 million Americans, said the researchers.
The researchers found that mice lacking the gene that
controls the production of type VI collagen developed osteoarthritis
at a rate more than five times greater than mice with a functioning
gene. Collagen is a ubiquitous protein found throughout the body in
connective tissue, muscle, cartilage and bone. To date, 27 different
types have been identified.
To examine structures within the cartilage of mouse
joints, Leonidas Alexopoulos, Ph.D., developed a novel "micro-vacuuming"
technique. With this device, Alexopoulos extracted key structures
within the cartilage of mouse hip joints, which are the size of the
ball in a ball-point pen, and analyzed how they responded to the stresses
of everyday life.
Alexopoulos presented the results of the Duke study
Feb. 20, 2005, at the 51th annual scientific meeting of the Orthopedic
Research Society in Washington, D.C. Alexopoulos, now a post-doctoral
fellow at the Massachusetts Institute of Technology, conducted the
research in the laboratory of Farshid Guilak, Ph.D., director of orthopedic
research and senior member of the Duke team. The study was funded
by the National Institutes of Health.
The researchers focused their attention on the narrow
region of tissue that surrounds the cartilage cells on the surface
of joints and is known as the pericellular matrix (PCM). Together
with cartilage cells known as chondrocytes, collagen types II, VI
and IX, and other proteins, the PCM forms a structure called a chondron,
which is believed to provide a "buffer" zone between the
cells and the remainder of the cartilage tissue.
"The interesting thing is that type VI collagen
occurs nowhere else in the cartilage but the PCM, and no one really
understood why," Alexopoulos explained. "When we analyzed
the PCM of mice unable to produce type VI collagen, we found that
the chondrons in these mice were much softer and the joints did not
respond well to mechanical pressures. The joint looked as if it osteoarthritis
had developed.
"It appears now that the type VI collagen acts
like a scaffold that provides structure and stiffness to the PCM,"
Alexopoulos continued. "With this model for osteoarthritis, we
have a better understanding of how changes in the mechanical forces
on the cells may lead to degeneration of the cartilage."
For their experiments, the team compared how chondrons
changed over time in three different groups of mice: one group had
functioning type VI collagen genes, while the two other groups were
strains of "knockout" mice developed by Paolo Bonaldo, University
of Padova, Italy.
One group of mice had both parents with the type VI
collagen gene knocked out, while the other group had only one parent
without the gene. After six months, the researchers removed chondrons
to determine how they responded.
"We found significant osteoarthritic and developmental
differences among the three groups," Alexopoulos said. Specifically,
73 percent of the mice with two knock-out parents showed evidence
of mild to severe osteoarthritis. This compared to 40 percent for
mice with one knock-out parent and 13 percent for the control mice.
"These findings represent an important advance
in our understanding of osteoarthritis," Guilak said. "The
study provides direct evidence of the role of type VI collagen in
the biomechanical properties of the PCM.
While the mechanism behind the accelerated development
of osteoarthritis is not yet clear, it suggests that the lack of type
VI collagen negatively impacts the ability of the cartilage to respond
properly to the mechanical stresses and pressures on the joint."
The experiments would not have been possible without
the custom-built "microaspirator," which could extract individual,
intact chondrons. Other methods of isolating chondrons, which either
involve dissolving surrounding tissues with harsh enzymes or grinding
the cartilage in pieces, typically yield damaged chondrons, Alexopoulos
said.
"Using a tiny syringe, I was able to go across
the surface of the cartilage and vacuum up the chondrons without damaging
them," Alexopoulos said. "The chondrons literally popped
out of the cartilage and into the syringe. From that point, it was
easy to analyze their structure."
It is estimated that more than 70 percent of Americans
over the age of 65 show some signs of osteoarthritis, which is characterized
by the slow degeneration of the buffering layer of cartilage within
joints. The other major form of arthritis, rheumatoid arthritis, occurs
when the body's immune system attacks the linings of joints.
Guilak currently leads of group of clinicians and
investigators from Duke and the Durham VA Medical Center who are carrying
out a broad range of basic and clinical research into better understanding
and treating osteoarthritis.
The effort is funded by grants from the National Institute
on Aging (NIA) and the National Institute of Arthritis and Musculoskeletal
and Skin Disease (NIAMS).
Contact: Richard Merritt
Merri006@mc.duke.edu
919-684-4148
Duke University
Medical Center
