MicroRNAs (miRNAs) are a group of endogenous non-coding RNAs that regulate gene expression. Alteration in miRNA expression results in changes in the profile of genes involving a range of biological processes, contributing to numerous human disorders. With high stability in human fluids, miRNAs in the circulation are considered as promising biomarkers for diagnosis, as well as prognosis of disease. In addition, the translation of miRNA-based therapy from a research setting to clinical application has huge potential. The aim of the current review is to: (i) discuss how miRNAs traffic intracellularly and extracellularly; (ii) emphasize the role of circulating miRNAs as attractive potential biomarkers for diagnosis and prognosis; (iii) describe how circulating microRNA can be measured, emphasizing technical problems that may influence their relative levels; (iv) highlight some of the circulating miRNA panels available for clinical use; (v) discuss how miRNAs could be utilized as novel therapeutics, and finally (v) update those miRNA-based therapeutics clinical trials that could potentially lead to a breakthrough in the treatment of different human pathologies.

Objective: To profile the expression of all known members of the matrix metalloproteinase (MMP), ADAMTS, and tissue inhibitor of metalloproteinases (TIMP) gene families in normal cartilage and cartilage from patients with osteoarthritis (OA).Methods: Human cartilage was obtained from femoral heads at joint replacement for OA or following fracture to the femoral neck. Total RNA was purified, and gene expression was assayed using quantitative real‐time polymerase chain reaction.Results: Several members of the above gene families were regulated in OA. Genes that showed increased expression in OA were MMP13, MMP28, and ADAMTS16 (all at P < 0.001), MMP9, MMP16, ADAMTS2, and ADAMTS14 (all at P < 0.01), and MMP2, TIMP3, and ADAMTS12 (all at P < 0.05). Genes with decreased expression in OA were MMP1, MMP3, and ADAMTS1 (all at P < 0.001), MMP10, TIMP1, and ADAMTS9 (all at P < 0.01), and TIMP4, ADAMTS5, and ADAMTS15 (all at P < 0.05). Correlation analysis revealed that groups of genes across the gene families were coexpressed in cartilage.Conclusion: This is the first comprehensive expression profile of all known MMP, ADAMTS, and TIMP genes in cartilage. Elucidation of patterns of expression provides a foundation with which to understand mechanisms of gene regulation in OA and potentially to refine the specificity of antiproteolytic therapies.

Summary: Replicative senescence occurs when normal somatic cells stop dividing. Senescent cells remain viable, but show alterations in phenotype, e.g. altered expression of matrix metalloproteinases (MMPs); these enzymes are known to be involved in cartilage destruction. It is assumed that cells deplete their replicative potential during aging, and age is a major risk factor for osteoarthritis (OA). Therefore, we hypothesized that chondrocytes in aging or diseased cartilage become senescent with associated phenotypic changes contributing to development or progression of OA.Articular cartilage was obtained from OA patients undergoing arthroplasty, with ‘normal’ cartilage from trauma surgery for hip fracture. Senescent cells were identified using the senescence‐associated β‐galactosidase (SA‐β‐gal) marker. Telomere length was assessed using Southern blot. MMP expression was measured at the mRNA level using Taqman® RT‐PCR.No SA‐β‐gal staining was observed in control cartilage regardless of patient age. In contrast, SA‐β‐gal staining was observed in damaged OA cartilage adjacent to the lesion. Cultured chondrocytes isolated from sites near a lesion contained a greater percentage of SA‐β‐gal positive cells than cultures isolated from distal sites or normal cartilage. Mean telomere length was shorter in cells near the lesion compared to distal sites in the same joint; thus the former population has undergone cell division. The expression of collagenases MMP‐1, ‐8 and ‐13 and tissue inhibitor of metalloproteinases (TIMP)‐1 was altered in OA cartilage, but no difference was detected between lesion and distal sites in the same joint (i.e. no correlation was found between senescent cells and proteinase/inhibitor expression).

Cartilage destruction in osteoarthritis (OA) is thought to be mediated by two main enzyme families; the matrix metalloproteinases (MMPs) are responsible for cartilage collagen breakdown, whereas enzymes from the 'a disintegrin and metalloproteinase domain with thrombospondin motifs' (ADAMTS) family mediate cartilage aggrecan loss. Tissue inhibitors of metalloproteinases (TIMPs) regulate the activity of these enzymes. Although cartilage destruction in OA might be driven by the chondrocyte, low-grade synovitis is reported in patients with all grades of this disease.Our earlier work profiling these gene families in cartilage identified a number of genes that are regulated in OA, which are hence implicated in the disease process. Because the synovium might contribute to cartilage-matrix destruction in OA, we have extended the screening in the current study. We have profiled MMP, ADAMTS and TIMP genes in both cartilage and synovium from patients with either OA of the hip or a fracture to the neck of femur (NOF), giving a more complete picture of proteolysis in this disease.The four most significantly upregulated genes (P < 0.0001) in OA synovium compared to the fractured NOF are MMP28, ADAMTS16, ADAMTS17 and TIMP2. For MMP9, MMP10, MMP12, MMP17, MMP23, MMP28, ADAMTS4, and ADAMTS9, there is a significant correlation between expression levels in the synovium and cartilage, suggesting similar mechanisms of regulation. Additionally, we have shown that in cartilage the median level of steady-state mRNA for MMP13 is approximately 20-fold higher than MMP28 and approximately 1,500-fold higher than ADAMTS16, with expression of this latter gene approximately 150-fold higher in synovium than cartilage.This study is the most comprehensive analysis of the metzincin family of proteinases in the joint to date and has identified several proteinase genes not previously reported to be expressed or regulated in synovium.

Objective: To use an in vitro model of chondrogenesis to identify microRNAs (miRNAs) with a functional role in cartilage homeostasis.Methods: The expression of miRNAs was measured in the ATDC5 cell model of chondrogenesis using microarray and was verified using quantitative reverse transcription–polymerase chain reaction. MicroRNA expression was localized by in situ hybridization. Predicted miRNA target genes were validated using 3′‐untranslated region‐Luc reporter plasmids containing either wild‐type sequences or mutants of the miRNA target sequence. Signaling through the Smad pathway was measured using a (CAGA)₁₂‐Luc reporter.Results: The expression of several miRNAs was regulated during chondrogenesis. These included 39 miRNAs that are coexpressed with miRNA‐140 (miR‐140), which is known to be involved in cartilage homeostasis and osteoarthritis (OA). Of these miRNAs, miR‐455 resides within an intron of COL27A1 that encodes a cartilage collagen. When human OA cartilage was compared with cartilage obtained from patients with femoral neck fractures, the expression of both miR‐140‐5p and miR‐455‐3p was increased in OA cartilage. In situ hybridization showed miR‐455‐3p expression in the developing limbs of chicks and mice and in human OA cartilage. The expression of miR‐455‐3p was regulated by transforming growth factor β (TGFβ) ligands, and miRNA regulated TGFβ signaling. ACVR2B, SMAD2, and CHRDL1 were direct targets of miR‐455‐3p and may mediate its functional impact on TGFβ signaling.Conclusion: MicroRNA‐455 is expressed during chondrogenesis and in adult articular cartilage, where it can regulate TGFβ signaling, suppressing the Smad2/3 pathway. Diminished signaling through this pathway during the aging process and in OA chondrocytes is known to contribute to cartilage destruction. We propose that the increased expression of miR‐455 in OA exacerbates this process and contributes to disease pathology.