MicroRNA-21 Mediates TGF-β-regulated Fibrogenic Activation Of Spinal Fibroblasts And The Formation Of Fibrotic Scars After Spinal Cord Injury

Traumatic spinal cord injury(SCI)is associated with a significantly high disability and mortality rate,with loss of sensory,motor,and autonomic functions.Currently,there is no effective treatment to promote axon regeneration and restore lost neurologic functions after SCI.Scar tissue is categorized into glial scars and fibrotic scars,and it is thought to be responsible for the failure of transected axons to regenerate in lesions following damage caused by traumatic SCI.Little regeneration of transected axons occurs after the damage caused by traumatic spinal cord injury,and unidirectional and irreversible fibrotic scars are thought to be the main chemical and physical obstacle for axonal regrowth in SCI pathology.Transforming growth factor(TGF)-βl is a fibrogenic factor that regulates fibroblast proliferation,differentiation,and apoptosis,and enhances the proliferation and extracellular matrix production of fibroblasts.MicroRNAs(miRNAs)are widely present in a range of tissues and fluids such as muscle,the skeleton,gray matter,plasma,serum,urine,and cerebrospinal fluid.miR-21 plays a major role in tissue pathological and physiological processes,and is an important regulator of molecular networks.Recent studies have revealed a role for miR-21 in the fibrosis of various tissues,including cardiac,lung,liver,kidney,tumor,and skin.We previously demonstrated that microRNA(miR)-21-5p and TGF-βl,a central pathological mediator of fibrotic diseases,were significantly up-regulated in the lesion epicenter after SCI.Here,we found that TGF-β1 enhanced miR-21-5p expression in primary spinal fibroblasts,and regulated the expression of fibrosis-related genes.The overexpression of miR-21-5p promoted the pro-fibrogenic activity of TGF-β1 in spinal fibroblasts,while miR-21-5p knockdown attenuated this activity.We identified Smad7 as a target gene of miR-21-5p,suggesting a potential mechanism for the role of miR-21-5p in spinal fibrosis through regulating Smad7 expression.Furthermore,miR-21-5p knockdown in a mouse model significantly improved motor functional recovery after spinal cord injury.These data demonstrate that miR-21-5p functions in an amplifying circuit to enhance TGF-β signaling events in the activation of spinal fibroblasts and suggest that miR-21-5p is a potential therapeutic target in the treatment of fibrotic scar formation after SCI.