MicroRNAs (miRs) are transcribed as stem-loop precursors harboring two different miRs on either side of the structure. Both miRs can modulate levels of cellular transcripts based on sequence complementarity between the miR and the mRNA target. The miR of the current study, miR-675, is encoded in the H19 gene with high expression in fetal/placental tissues but low levels in most adult tissues except for skeletal muscle and articular cartilage. miR-675 has a supportive role in expression of the major collagen component of articular cartilage (COL2A1) but it is unknown which arm contributes to this effect. Objectives: To determine the active arm of miR-675 in human articular chondrocytes. To evaluate effects of overexpression of both arms of miR-675 on MMP1 and MMP13, two enzymes involved in breakdown of COL2A1. To investigate whether abundance of both arms of miR-675 is dynamic.
miR-arm activity was determined by association with the AGO2 complex using immunoprecipitation with an AGO2 specific antibody. miR overexpression and inhibition was used to identify indirect downstream effects on two targets of the Matrix-Metalloprotease family, MMP1 and MMP13. Data was evaluated by qPCR and enzymatic activity assays. Early passage human articular chondrocytes (up to passage 2) obtained from cartilage from both healthy and osteoarthritis affected tissue were used. To evaluate miR-675 levels in a different model, myotube differentiation was employed.
We show that both arms of miR-675 have opposing effects on MMP1 and MMP13; however only one arm, miR-675-3' is active in human articular chondrocytes. We demonstrate that during myotube differentiation, high expression of both arms of miR-675 is observed as well as an increase in expression of MMP1.
We show that both arms of miR-675 result in opposing effects on two downstream molecules MMP1 and MMP13. We propose that miR abundance may arise as response to direct target transcript levels and are thus dynamic to meet the requirements of the cellular environment.