let-7i Inhibition Potentiates Progesterone-Inuduced Functional Recovery in a Mouse Model of Ischemia

Background: Progesterone (P4) is a potent neuroprotectant and a promising therapeutic for stroke treatment. However, the underlying mechanism(s) remain unclear. One known mediator of P4’s protective function is brain-derived-neurotrophic-factor (BDNF). Of note, we recently reported that P4 induces a significant release of BDNF from primary astrocytes, through progesterone-receptor-membrane-component-1 (Pgrmc1). This receptor is abundantly expressed in the brain and mediates various beneficial effects of P4 including anti-apoptosis spinogenesis, and BDNF release. What is not known, however, is how the expression of this receptor is regulated. Purpose: This study was aimed to elucidate what regulates the expression of Pgrmc1 and BDNF in glia and how such regulation influences the neuroprotective function of P4 in the ischemic brain. We hypothesized that let-7i represses P4’s neuroprotective effects by down-regulating the expression of both Pgrmc1 and BDNF in glia, leading to: 1) suppression of P4-induced BDNF release from glia, and 2) attenuation of the beneficial effects of P4 on neuronal survival and markers of synaptogenesis in the ischemic brain. Methods: Primary cortical astrocytes and neurons were used as experimental models to investigate the role of let-7i in P4’s action in vitro. For in vivo experiments, we induced stroke using the middle cerebral artery occlusion (MCAo) method in ovariectomized mice (a model of surgical menopause). Let-7i expression was manipulated using a let-7i inhibitor, delivered via intracerebroventricular (ICV) injection. Results: Our data suggest that the microRNA (miRNA), let-7i, is a negative regulator of Pgrmc1 and BDNF in glia, and that let-7i disrupts P4-induced BDNF release and P4’s beneficial effects on cell viability and markers of synaptogenesis. Results from our in vivo experiments revealed that inhibiting let-7i enhances P4 induced neuroprotection and facilitates functional recovery following stroke. Conclusions: Collectively, the data presented here suggested that in the ischemic brain, let-7i negatively influences P4-induced neuroprotection via regulation of the Pgrmc1/BDNF axis. As such, inhibition of let-7i may be an effective means to enhance the efficacy of P4 in treating ischemic stroke. In addition, the discovery of such factors that regulate the cytoprotective effects of P4 may lead to the development of biomarkers to differentiate/predict those likely to respond favorably to P4 versus those that do not.