miR-125/CDK2 axis in cochlear progenitor cell proliferation

Hearing loss ranks fourth among the principal causes of disability worldwide, and manipulation of progenitor cells may be a key strategy for hair cell regeneration. The present study investigated the role and mechanism of miR-125 on the proliferation of cochlear progenitor cells (CPCs). CPCs were isolated from the cochleae of neonatal rats, and their morphology was observed. Furthermore, the differentiation ability of CPCs was determined by assessing the expression of 5-bromodeoxyuridine (BrdU), nestin and myosin VII by immunofluorescence. The expression levels of miR-125 and cyclin-dependent kinase 2 (CDK2) as well as the cell proliferation of CPCs were assessed. In addition, following gain- and loss-of-function assays, the cell cycle was examined by flow cytometry, and the expression levels of miR-125, CDK2, proliferating cell nuclear antigen (PCNA) and nestin were determined by reverse transcription-quantitative PCR and western blotting. The binding sites between miR-125 and CDK2 were predicted by TargetScan and identified by the dual luciferase reporter assay. The results demonstrated that different types of progenitor spheres were observed from CPCs with positive expression of BrdU, nestin and myosin VII. Following in vitro incubation for 2, 4 and 7 days, the spheres were enlarged, and CPC proliferation gradually increased and reached a plateau after further incubation for 3 days. Furthermore, the expression levels of nestin and PCNA in CPCs increased and then decreased during in vitro incubation for 2, 4 and 7 days. Following this incubation, the expression levels of miR-125 in CPCs decreased; thereafter, its expression increased, and the expression pattern was different from that of CDK2. In addition, miR-125 overexpression in CPCs decreased the expression of CDK2 and the number of cells in the S phase. Different expression patterns were found in CPCs in response to the miR-125 knockdown. In addition, miR-125 directly targeted CDK2. Simultaneous knockdown of miR-125 and CDK2 enhanced CPC proliferation compared with CDK2 knockdown alone. Taken together, the findings from the present study suggested that miR-125 may inhibit CPC proliferation by downregulating CDK2. The present study may provide a novel therapeutic direction for treatment of hearing loss.