On the Horizon From the ORS

Abstract

How Do Synovial Joints Come About? Synovial joints are easily injured and can lose function with age or because of chronic conditions. Significant advances have been made in understanding the joint structure, biomechanics, and biology of articular cartilage and other joint tissues. However, this progress has not been accompanied by equivalent advances in our understanding of how synovial joints form in the fetus and come to acquire their structuralfunctional properties. Arguably, an in-depth understanding of the developmental biology of synovial joints could lead to novel and superior therapies. If we knew what “master” genes dictate formation of articular cartilage, we could use those genes (or drugs that positively affect them) to boost or restore joint function in the elderly. Similarly, if we were to identify joint-forming progenitor cells and characterize their mechanisms, we could engineer stem cells and endow them with joint-repair and regeneration capacity. The authors of three studies made use of genetic cell tracing and tracking strategies to identify progenitor cells responsible for synovial joint formation during limb skeletogenesis.1-3 Prospective joint sites in early fetal limbs contain mesenchymal cells, collectively known as the interzone, that are characterized by expression of Gdf5, a member of the bone morphogenetic protein (BMP) family. In these studies, transgenic mice expressing Cre recombinase under Gdf5 regulatory gene sequences (Gdf5-Cre) were mated with Rosa R26R LacZ reporter mice so that the joint site-associated mesenchymal cells in resulting double transgenic Gdf5-Cre/Rosa mice became LacZ positive; thus, their behavior, function, and fate could be monitored during prenatal and postnatal joint development. Strikingly, the LacZ positive cells remained localized and restricted to the joint area and over time gave rise to articular cartilage, intrajoint ligaments, and synovial lining. The cells, however, were absent from the remaining portions of the long bone elements; thus, they were not part of the growth plates and did not participate in bone formation. The data reveal for the first time that the Gdf5-expressing and LacZ-positive interzone cells constitute a specialized cohort of progenitor cells with the innate and exclusive capacity to form synovial joints. How can such a population of progenitor cells give rise to both articular cartilage and noncartilaginous tissues within the joint? Possible answers to this critical question lie in previous and seemingly puzzling studies showing that developing joints express a plethora of both chondrogenic and antichondrogenic factors, including BMP, the polypeptide noggin, transforming growth factor-β, the Wnt family of proteins, parathyroid hormone-related protein, the transcription factor cut-like homeobox 1, and the transcription factor Erg.4-8 These factors are likely to act in specific spatiotemporal ways. The chondrogenic factors could induce a subset of the joint progenitor cells to differentiate into articular chondrocytes and establish their permanent phenotype. The antichondrogenic factors would have more complex roles. They would be essential at the very onset of joint formation to establish the initial mesenchymal character of the entire interzone. Their action would then become more restricted and directed toward different subsets of progenitor cells to promote their development into nonchondrogenic joint tissues. Koyama et al2 provided in vitro evidence that interzone cells are developmentally malleable and can indeed be steered toward a chondrogenic or nonchondrogenic phenotype by BMP and Wnt signaling, respectively. Some progress has been made, but much remains to be uncovered. For instance, articular cartilage is an anisotropic tissue9 in which the superficial layer is made of flat lubricin-producing cells and the remainder contains distinct zones of round chondrocytes. Synovium and intrajoint ligaments are markedly different tissues. The ligaments themselves have zonal specification. In addition, each joint is morphologically unique, and the opposing sides of each joint have reciprocally shaped configurations. It will require major research efforts to clarify how the joint-forming progenitor cell population accomplishes such remarkable differentiation and morphogenetic feats. Such information could bring about important future joint therapies. Maurizio Pacifici PhD Seeking Pain Relief in Osteoarthritis Both mechanical and pharmacologic methods are employed in the nonsurgical treatment of osteoarthritis (OA). Reduction of mechanical forces across the joint likely decreases the rate of cartilage loss, and improved muscle strength is generally accepted as a way to provide shock absorption for articular cartilage. However, it is not clear why the perception of pain may be immediately ameliorated by the application of a brace or of shock-absorbing shoes. With the discovery of salicylates, a long string of anti-inflammatory drugs has been marketed for pain management of OA. In most persons, the long-term anti-inflammatory effect on a joint is not as important as is the more immediate perception of pain relief caused by lower levels of prostaglandin E2 (PGE2). PGE2 affects local pain receptors and neurotransmitters in both the spinal cord and brain. Acetaminophen affects different pathways and offers few side effects in healthy individuals. Drugs that affect opioid receptors are very effective but have side effects that include the potential for accommodation and addiction. Neutriceuticals such as glucosamine and chondroitin sulfate appear to have an effect on patients with moderate to marked OA.10 Given the minuscule changes in joint fluid level, the mechanism of action of glucosamine and chondroitin likely lies outside the joint. The predominant effects of OA on quality of life and function are related to pain. Safer pain management without side effects will be important in future treatment. A better understanding of the origins of joint pain likely will be a mainstay in the management of OA and postsurgical pain. Two elements are required to find the best solutions: determining the location of joint pain receptors and the important mediators for these receptors, and defining the long-term beneficial and adverse effects of modifying joint pain. Regarding the location of joint pain receptors and their mediators, Bergstrom et al11 demonstrated, based on animal studies, that opioid receptors exist in joint capsule and periosteum. These authors found enkephalins to be associated with substance P. Substance P is an 11-amino-acid neuropeptide that acts as one of many neurotransmitters. In a study on knee joint arthroplasty surgery, the presurgical concentration of substance P levels accurately predicted postoperative pain relief satisfaction.12 Ogino et al,13 in an unpublished study, reported having located class III β-tubulin (TuJ1) and substance P in osteoarthritic bone cysts. It appears from this and other work that a variety of joint tissues is involved in OA pain. In addition, clinicians have noted divergent patient responses to constant passive motion (CPM) following knee joint arthroplasty. In a recent ORS presentation, Okamoto et al14 used CPM with electromyograms in rats and found that differences in the levels of adenosine triphosphate, bradykinin, acetylcholine, and serotonin are responsible for various clinical responses. (Some patients with joint replacement surgery feel better with CPM; others feel worse.) The authors related these differences to those in resting pain and motion pain in human OA. (Some patients with OA like being still; others prefer motion.) The excitatory amino acids (EAAs) glutamate and aspartate are also important neurotransmitters in OA.15 The reduction in levels of EAAs that occurs with injection of hyaluronic acid may explain the immediate pain relief seen in some individuals. In a rabbit model of anterior cruciate ligament disruption, levels of the EAAs glutamate and aspartate were increased by approximately 200%, and their receptors were increased in chondrocytes.16 The role of EAAs in OA progression is proposed but not proved. Furthermore, unrelated to joints, radiofrequency treatment in tendons results in a significant reduction in TuJ1 and calcitonin gene-related peptide nerve fibers in the tissue. The authors feel that this may be a mechanism in postoperative pain relief following microtenotomy.17 Modification of joint pain must be balanced against the potential for increasing the rate of articular cartilage degeneration. Pain relief may result in gait changes that would be counterproductive to joint maintenance.18 Any agent that is found to reduce OA pain must be monitored to see whether long-term effects are beneficial or harmful to the joint. The clinician will need to keep a wary eye on what may appear at first to be very promising results. Fred R.T. Nelson MD Minimally Invasive In Vivo Bone Material Property Measurement Bone material properties could be a factor in fracture risk, but how can these properties be measured in patients? Certainly it is not practical to cut conventional mechanical testing specimens from the bones of living persons. Could a noninvasive or minimally invasive instrument be developed to measure such properties? At least two efforts are ongoing to develop such an instrument. One group of investigators uses Raman spectroscopy, through the skin,19 to measure spectral features such as bone's carbonate:phosphate ratio. This ratio is a potential biomarker for fragility fracture susceptibility in osteoporotic bone tissue.20 This technique has been applied to image the composition of bone tissue within an intact section of a canine limb.21 Now available commercially, the bone diagnostic instrument (BDI)22 (Active Life Technologies, Santa Barbara, CA) uses indentation testing to determine bone material properties in vivo. The device has progressed through 18 prototypes; in the Osteoprobe II BDI, the probe assembly, which is designed to penetrate soft tissue, consists of a reference probe (a 22-gauge hypodermic needle) and a test probe (a small-diameter, sharpened rod), which slides through the inside of the reference probe.23 The probe assembly is inserted through the skin to rest on the bone. The distance that the test probe is indented into the bone can be measured relative to the position of the reference probe, which rests on the surface of the bone. At this stage of development, the indentation distance increase, with repeated cycling to a fixed force, appears to best distinguish bone that is easily fractured from bone that is less easily fractured.23 Time and clinical trials will determine whether these instruments can contribute significantly to the assessment of fracture risk and the development and monitoring of treatment. Paul K. Hansma PhD