THE SONIC HEDGEHOG AGONIST SAG ATTENUATES MITOCHONDRIAL DYSFUNCTION AND DECREASES THE NEUROTOXOCITY INDUCED BY FRATAXIN-DEFICIENT ASTROCYTES

Abstract

Friedreich’s ataxia (FRDA) is predominantly a neurodegenerative disease caused by the deficiency of a protein called frataxin (FXN). Although the main pathological alterations are observed in neurons, it is becoming clear that other non-neuronal cells such as astrocytes may be actively involved in the neurodegenerative process associated with the disease. Depending on the stimuli they respond to, astrocytes acquire different activation states in a process called astrogliosis. Neuroinflammatory stimuli induce the formation of A1 reactive astrocytes, which upregulate proinflammatory genes, being harmful for neurons. A1 astrocytes have been detected in post-mortem tissue of patients with different neurodegenerative disorders, being hypothesized that they might have deleterious effects on neurons, exacerbating the neurodegenerative process. Recent studies have demonstrated positive effects of Sonic Hedgehog (SHH) agonists in astrocyte viability and proliferation, astrocyte-mediated neuroprotection, and also positive effects in mitochondrial activity and dynamics. As mitochondrial changes are important components in the etiology of neurodegenerative disorders, the influence of SHH agonists in mitochondrial physiology could be of therapeutic relevance. In this work, we have thoroughly characterized astrocyte reactivity phenotype and mitochondrial status of FXN-deficient human astrocytes, evaluating as well the effect of SHH agonists on astrocyte reactivity, viability, and function. We used an in vitro model based on a short hairpin RNA packaged in a lentiviral vector, which allowed us to decrease FXN levels in human cortical astrocytes, to similar levels as those observed in FRDA patients, and found that FXN-deficient cells had less cell viability and higher expression of several A1 reactive astrocyte markers, than control cells. Both phenomena were prevented by a chronic treatment with the smoothened agonist (SAG), a SHH signaling agonist. Moreover, FXN-deficient astrocytes showed defects in mitochondrial function and dynamics, which were partially rescued by SAG. Regarding the possible neuroprotective effects of SHH agonists, previous results showed that FXN-deficient astrocytes are able to induce neurodegeneration, and we have observed that the chronic treatment with SAG attenuated the neurotoxicity triggered by the treatment of mouse cortical neurons with conditioned medium of FXN-deficient astrocytes. Overall, our results suggest that the treatment of FXN-deficient astrocytes with a SHH agonist like SAG, could be used as a possible target to reduce FRDA-associated neurodegeneration.