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The stability of the lattice-like type II collagen architecture of articular cartilage is paramount to its optimal function. Such stability not only depends on the rigidity of collagen fibrils themselves, but more importantly, on their interconnections. One known interconnection is through type IX and biglycan molecules. However, the mechanical properties of this interaction and its role in the overall stability remain unrevealed. Using atomic force microscopy, this study directly measured the mechanical strength (or the rupture force) of a single bond between collagen IX and biglycan. The results demonstrated that the rupture force of this single bond was 15pN, which was significantly smaller than those of other known molecule interactions to date. This result suggested that type IX collagen and biglycan interaction may be the weak link in the cartilage collagen architecture, vulnerable to abnormal joint force and associated with disorders such as osteoarthritis.

Decorin is one important member of the family of small leucine-rich proteoglycans, which are widely distributed in connective tissues in the body such as tendon and ligament. Decorin may be responsible for collagen fibril connection in those tissues. A recent hypothesis suggests that decorin may bind to collagen with its core protein while binding to another decorin through the interaction with their glycosaminoglycan (GAG) chains. However, there is no direct evidence supporting this hypothesis to date. In this study, the interaction of decorin GAG chains was directly determined for the first time. The rupture force of single bonds between decorins (GAG chains interaction) was determined directly as 16.5+/-5.1 pN using a laser tweezers/interferometer single molecular nanomechanical testing system. This information can improve our understanding of the mechanical properties of connective tissues at the molecular level.

The purpose of this study was to examine whether mechanical tensile forces affect estrogen regulation of collagen synthesis of anterior cruciate ligament fibroblasts at the mRNA level. Estrogen was studied at three physiologic levels, 10(-11), 10(-10), and 10(-9)M. The results revealed that estrogen alone stimulated Type I and III collagen synthesis at the mRNA level, and application of mechanical force decreased the expression of collagen Type I and III genes at all tested estrogen levels. These findings suggest that estrogen may directly regulate ligament structure and function by alteration of Type I and III collagen synthesis. This regulation is dependent on mechanical loading.

Proteoglycan aggregate is a major component of the extracellular matrix in articular cartilage and is considered to be responsible for the resistance to compression of this tissue. The reduced stiffness of articular cartilage due to the loss of proteoglycan aggregate has been reported in osteoarthritis. In order to understand the mechanical properties of extracellular matrix in articular cartilage at molecular level, the compressive properties of 36 single molecules of proteoglycan aggregate were directly measured using a laser tweezers/interferometer system. The proteoglycan aggregates showed resistance when compressed to approximately 30% of their contour length. The stiffness of proteoglycan aggregates increased non-linearly from 2.6+/-3.8 pN/microm (compressed to 30-35% of their contour length) to 115.5+/-30.9 pN/microm (compressed to 2.5-5% of their contour length).

This study presents a method for direct measurement of the compressive properties of single molecules of proteoglycan aggregate using a state-of-the-art laser tweezers/interferometer system previously developed to test the tensile properties of single molecules. A typical molecule of proteoglycan aggregate showed a highly non-linear resistance to compression after being compressed to about 25% of its original molecule length.

Collagens are the most abundant structural proteins found in the extracellular matrix of vertebrates. Knowledge of the mechanical behavior of collagen monomers is essential for understanding the mechanical properties of collagen fibrils that constitute the main architectural framework of skin, bone, cartilage, and other connective tissues. In this study, the flexibility of type I collagen monomer was studied by stretching type I collagen monomers directly. The force-extension relationship was measured and analyzed by fitting the data into a worm-like chain elasticity model. The persistence length of collagen I monomer was determined to be 14.5 nm and the contour length was 309 nm. The results confirm that type I collagen monomer is flexible rather than rigid, rod-like molecule. Such flexibility may possibly be a consequence of the micro-unfolding of discrete domains of single collagen molecule.

Optical tweezers is a useful technique to study single molecules. It has been applied to stretch biopolymers in several approaches. However, these approaches may not be appropriate to measure short biopolymers. In the present study, the optical signals of beads in different situations were studied. A new method was developed for stretching short biopolymers, as short as 300 nm in length. This new method was successfully applied to determine the stiffness of procollagen I molecules., (Copyright 2001 Academic Press.)

This study developed a micromechanical test method for determination of stiffness of collagen molecules. The novelty includes the adoption of an optical tweezer system for loading a single collagen molecule and the development of a binding technique for gripping the collagen molecule termini for the micromechanical test. This methodology will potentially help us to understand the mechanical properties and functions of collagen ultrastructure.