Behavior of ordered sodium in enzymatically depleted cartilage tissue

Abstract

The onset of cartilage tissue disorders can be characterized by a loss of proteoglycans (PGs) and diagnosed by contrast-enhanced proton ( 1 H) MRI techniques, as well as sodium MRI. The behavior of sodium located in anisotropic environments, is examined as a function of cartilage degeneration. PGs are proteolytically depleted from the cartilage samples, which gives rise to a decrease of the ordered sodium content. More surprisingly, however, the residual quadrupolar couplings are shown to increase with increasing depletion levels. Since the residual quadrupolar couplings are intimately related to local order and anisotropic motion, measuring their distribution in cartilage may provide insight into the structural changes that occur within the tissue upon degradation. In this study relatively mild orientational dependence of the couplings was found. Little or no free sodium was observed in the cartilage specimens under study. Articular cartilage is a dense connective tissue composed primarily of water, collagen fiber bundles, and proteoglycan (PG) aggregates. Cartilage functions as the load-bearing surface in the joints of the musculoskeletal system. Modern developments in chondroprotective therapies, gene therapy, cartilage grafting, and tissue engineering have increased the demand for sensitive, reliable, and noninvasive techniques that can enable the detection of biochemical changes in cartilage in vivo.The major macromolecules in cartilage are collagen type II and PGs. PGs provide much of the compressive stiffness through electrostatic repulsion, and collagen provides tensile and shear strength. Several studies have shown that the early stage of osteoarthritis (OA) is primarily associated with a loss of PGs, with minor changes in collagen structure and content (1-3). PGs consist of a central protein core to which a large number of negatively charged glycosaminoglycan (GAG) side chains are covalently attached. The GAG molecules consist of long-chain unbranched repeating dimeric polysaccharides, the most common of these being chondroitin-4-sulfate, chondroitin-6-sulfate, and keratan sulfate. Their negatively-charged side groups (sulfates and carboxylates) draw sodium ions into the framework to maintain overall electroneutrality. The swelling pressure generated by sodium ions allows cartilage to maintain its form and durability.Conventional proton ( 1 H) MRI techniques, such as T 1 , T 2 , and magnetization transfer (MT)-weighted imaging, have been shown to be inconclusive in detecting early changes in OA (4 -6). Recent work on delayed gadolinium (Gd)-enhanced MRI of cartilage (dGEMRIC) (7,8) and sodium MRI (9,10) have shown that it is possible to measure PG changes in cartilage both in vivo and in vitro. Although the dGEMRIC method offers certain advantages, it requires the intravenous injection of contrast agent, a significant delay before a T 1 map can be obtained, and joint exercise after the injection of the contrast agent for better penetration of Gd (DTPA) 2-into the cartilage. Also, there is a need for ac...

Assessment of glycosaminoglycan concentration in vivo by chemical exchange-dependent saturation transfer (gagCEST)

“…1 b and d). This difference indicates a more restricted motion of GAG in cartilage because it is anchored on the collagen fibrils, which is consistent with reports from 13 C NMR spectroscopy (35). Moreover, as with the case of -NH of GAG, NOE is likely to contribute to the low efficiency of some of the other -NH CEST-based applications in vivo (22, ¶).…”

“…As discussed above, the negative NOE observed in this work results from the interaction of water and nonexchangeable GAG protons with low mobility such that 0 c Ն 1. c of GAG in cartilage was reported to be Ϸ50 ns as measured by 13 C spectroscopy (35), which makes 0 c Ͼ Ͼ 1. Although c of the dipolar interaction between the GAG protons and the water protons is shorter than that of 13 C, 0 c Ն 1 is likely to hold at lower magnetic fields, which implies that NOE from GAG should contribute substantially to CEST/magnetization transfer on a 1.5-T clinical scanner.…”

“…With the advent of high-field MRI scanners, 23 Na MRI gains much attention because it becomes possible to use the concentration of positively charged 23 Na in cartilage to map the negatively charged GAGs (11,12). The quadrupolar coupling of 23 Na in cartilage has also been reported to correlate with degeneration (13,14). This effect could be used in combination with quadrupolar contrast techniques to elucidate degradation processes (15)(16)(17)(18).…”

Clean demarcation of cartilage tissue 23Na by inversion recovery

“…In the particular case of cartilage tissue, the difference is especially large due to immobilization and larger induced quadrupolar interactions of the bound sodium. It is possible that cartilage-tissue also contains free sodium, as one could expect in analogy to experiments performed with 2 H [17,18], but we have not seen a significant fraction thereof in bulk measurements [15].…”

“…More recently, methods based on frequency-sweep pulses [11,12] and quadrupolar nutation [13] were demonstrated, which exploit coherence transfer properties that depend on the quadrupolar interaction. The quadrupolar coupling itself was shown to correlate with the onset of cartilage degeneration [14,15].…”