Calling time on osteoarthritis

Osteoarthritis is a painful and disabling condition of the joints affecting more than 6 million people in the UK alone, with no cure currently available. Here, Nicole Gossan explains how new research into body clocks could lead to novel ways of managing the condition and open up exciting new therapeutic avenues

Osteoarthritis (OA) is a painful and disabling joint disease, caused by the degeneration of the cartilage normally cushioning the ends of bones. It is the most common joint disorder, affecting around 6 million people in the UK, but the mechanism is poorly understood and treatment options are limited to painkillers and eventual joint replacement. OA occurs when cartilage tissue degenerates, as a result of multiple interacting factors including genetics, obesity, mechanical injury and aging.

Healthy cartilage tissue comprises an abundant extracellular matrix and one specialised cell-type, the chondrocyte. The majority of the tissue is made up of matrix which provides a strong, lubricated surface at the ends of bones, dissipating shear and compressive forces and allowing joints to articulate smoothly. Because adult chondrocytes very rarely divide, the existing pool of cells is solely responsible for upkeep of the tissue and a fine balance must be achieved between activities which build up and break down matrix components. In OA, this balance is disturbed and chondrocytes excessively degrade their surrounding matrix, leading to tissue destruction and ultimate loss of joint function. The symptoms of OA, which include pain and joint stiffness, are often worse at certain times of day. This suggests that body clocks may play a role in the disease, which is the focus of new research from the University of Manchester.

The body clock (aka the circadian clock) is an intrinsic timekeeping mechanism used by the body to coordinate physiology, metabolism and behaviour to the external 24 h day/night cycle. In mammals such as humans, this process is controlled by an area of the brain called the suprachiasmatic nuclei (SCN). The SCN integrates time-cues from the environment (mainly light) and transmits this information to almost every cell and tissue in the body.

Different tissues use this timing information to align their activity, whether that is metabolism by the liver or the rate of contraction of the heart, together ensuring that an organism’s activity is adapted to the time of day at which it is active. It sometimes takes the body a few days to catch up with large shifts in external environment, such as those experienced following a transatlantic flight, which explains the phenomenon of jetlag. Body clock disruption can occur through a combination of genetic disposition and lifestyle factors in humans, and is modelled in the lab using model organisms such as mice. Such disruption has been associated with a wide range of health problems, for instance diabetes, neurodegenerative disease, metabolic disorder and cancer.

Single cells contain circadian clocks. At the molecular level, the “hands” of the clock consist of interlocking feedback loops of transcription factors . The core loop consists of CLOCK and BMAL1 proteins, which act together to drive expression of a family of genes. These target genes are different in each tissue, allowing regulation of tissue specific functions. Among these targets are the PERIOD (PER) and CRYPTOCHROME (CRY) proteins, which feedback to inhibit CLOCK and BMAL1 action. CLOCK and BMAL1 suppression leads to a reduction in PER and CRY, eventually reducing their feedback effect and allowing the cycle to begin again. This process takes approximately 1 day to complete, and can be measured in the lab using a clever combination of mouse genetics and bioluminescence imaging. Luciferase (luc) is an enzyme expressed in fireflies which produces bioluminescence in the presence of its substrate luciferin and an energy source. In 2004, Yoo et al.1 created a transgenic reporter mouse in which the PER2 protein was fused to luciferase (PER2::luc). Resultant light production faithfully reflects PER2 protein production, allowing real-time quantification of protein level and of circadian dynamics in tissues and cells from the mouse. This technology was adopted by researchers from the University of Manchester to establish the presence of a circadian clock in cartilage tissue, and investigate how the clock changes during aging and what this might mean for OA sufferers2.

Using cartilage cultures from PER2::luc mice, they found that the clock could be reset by hormone and temperature signalling, possible ways in which the brain could communicate time-of-day information to this tissue. In aged animals, the strength (or amplitude) of the circadian clock in cartilage was reduced by 40%. To gain some insight into which genes and processes might be circadian in cartilage (and in turn affected by age-related clock deterioration) the researchers next used microarray technology. Microarrays allow the simultaneous measurement of all the genes expressed in any tissue sample.

By measuring gene expression at multiple timepoints across two days, they identified 615 genes which were expressed in a circadian manner. This corresponds to almost 4% of all the genes expressed in cartilage. Oscillating genes were responsible for cellular processes including apoptosis (cell-death) and the breakdown of protein molecules. Changes to these processes during aging may well underlie OA development. Indeed, several genes which have previously been linked to OA were identified as circadian. These included genes from the Adamts family of proteases, which break down matrix components, and a recently identified OA susceptibility gene Gnl3.

To investigate whether the circadian clock changes during OA initiation, the researchers collaborated with a group at the University of Melbourne. The Australian group were experts at the surgical induction of experimental OA in mice, and had recently used microarrays to compare the genes expressed in healthy joint cartilage vs the earliest steps of OA initiation and progression. The researchers from Manchester analysed this data and demonstrated that core circadian clock genes were changing during disease initiation, suggesting that this could contribute to disease onset through misregulation of downstream clock-controlled genes.

Given that the circadian clock in aged cartilage is weaker, these findings could partially explain why OA incidence increases in the elderly. Age is one of the major risk factors for OA development, but not all old people develop the disease. There is therefore great interest in the changes occurring in the cartilage during aging, and how these changes may provide a platform upon which other risk factors interact in disease initiation. Importantly, the decline in the circadian clock during aging can potentially be fixed. Researchers found that applying a synthetic glucocorticoid to aged tissue cultures restored their circadian clock function, and while this does not offer a realistic treatment due to the extensive off-target effects of glucocorticoids in the body, it does offer proof of the concept. Several factors are experimentally known to strengthen the circadian clock, including scheduled feeding and exercise. In people, this would amount to sticking to scheduled meal-times, exercising at regular times of the day and having regular bedtimes. Environmental light is also important; bright light of certain wavelengths optimally signal to the clock in the SCN, and thus should be avoided at night. Lifestyle changes such as these would not only strengthen the circadian clock of the cartilage but that of the whole body, leading to general health and cognitive benefits. However, if such a regime sounds impractical then all is not lost. Several labs are working on identifying and developing compounds to selectively target the circadian clockwork. Further, existing or new therapies to provide pain relief or to treat OA may well benefit from time-of-day dependent administration.

Author: Nicole Gossan is currently writing up her PhD on the role of the cartilage circadian clock in aging and disease at the University of Manchester.

References: 1. Yoo SH et al. (2004) PERIOD2::LUCIFERASE real-time reporting of circadian dynamics reveals persistent circadian oscillations in mouse peripheral tissues, Proc Natl Acad Sci USA, 101(15): 5539-46 2. Gossan et al. (2013) The circadian clock in chondrocytes regulates genes controlling key aspects of cartilage homeostasis, Arthritis and Rheumatism