We have developed a mouse in which the Cre recombinase gene has been targeted to exon 1 of the matrilin-1 gene (Matn1) to investigate the origins of articular chondrocytes and the development of the knee joint. Analysis of joints from offspring of Matn1-Cre/R26R crosses demonstrated that articular chondrocytes are derived from cells that have never expressed matrilin-1 whereas the remainder of the chondrocytes in the cartilage anlagen expresses matrilin-1. A band of chondrocytes adjacent to the developing interzone in the E13.5 day knee joint became apparent because these chondrocytes did not turn on expression of matrilin-1 in contrast to the other chondrocytes of the anlagen. The chondrocytes of the presumptive articular surface therefore appear to arise directly from a subpopulation of early chondrocytes that do not activate matrilin-1 expression rather than by redifferentiation from the flattened cells of the interzone. In addition, lineage tracing using both Matn1-Cre/R26R and Col2a1-Cre/R26R lines indicated that non-cartilaginous structures in the knee such as cruciate ligament, synovium and some blood vessels are formed by cells derived from the early chondrocytes of the anlagen.

The knee joint consists of multiple interacting tissues that are prone to injury-and disease-related degeneration. Although much is known about the structure and function of the knee's constituent tissues, relatively little is known about their cellular origin and the mechanisms governing their segregation. To investigate the origin and segregation of knee tissues in vivo we performed lineage tracing using a Col2a1-Cre/R26R mouse model system and compared the data obtained with actual Col2a1 expression. These studies demonstrated that at E13.5 the interzone at the presumptive joint site forms when cells within the Col2a1 -expressing anlagen cease expression of Col2a1 and not through cellular invasion into the anlagen. Later in development these interzone cells form the cruciate ligament and inner medial meniscus of the knee. At E14.5, after interzone formation, cells that had never expressed Col2a1 appeared in the joint and formed the lateral meniscus. Furthermore, cells with a Col2a1 -positive expression history combined with the negative cells to form the medial meniscus. The invading cells started to express Col2a1 1 week after birth, resulting in all cells within the meniscus synthesizing collagen II. These findings support a model of knee development in which cells present in the original anlagen combine with invading cells in the formation of this complex joint.

Key Points This study has identified a novel mechanism by which TF expression is posttranscriptionally regulated in macrophages. The mechanism involves the control of mRNA stability by a cooperation between PARP-14 and TTP.

We aimed to develop a quantitative antibody-based near infrared fluorescence (NIRF) approach for the imaging of oxidized LDL in atherosclerosis. LO1, a well- characterized monoclonal autoantibody that reacts with malondialdehyde-conjugated LDL, was labeled with a NIRF dye to yield LO1-750. LO1-750 specifically identified necrotic core in ex vivo human coronary lesions. Injection of LO1-750 into high fat (HF) fed atherosclerotic Ldlr−/− mice led to specific focal localization within the aortic arch and its branches, as detected by fluorescence molecular tomography (FMT) combined with micro-computed tomography (CT). Ex vivo confocal microscopy confirmed LO1-750 subendothelial localization of LO1-750 at sites of atherosclerosis, in the vicinity of macrophages. When compared with a NIRF reporter of MMP activity (MMPSense-645-FAST), both probes produced statistically significant increases in NIRF signal in the Ldlr−/− model in relation to duration of HF diet. Upon withdrawing the HF diet, the reduction in oxLDL accumulation, as demonstrated with LO1-750, was less marked than the effect seen on MMP activity. In the rabbit, in vivo injected LO1-750 localization was successfully imaged ex vivo in aortic lesions with a customised intra-arterial NIRF detection catheter. A partially humanized chimeric LO1-Fab-Cys localized similarly to the parent antibody in murine atheroma showing promise for future translation.