The smoothened agonist SAG reduces mitochondrial dysfunction and neurotoxicity of frataxin-deficient astrocytes

Abstract

Background Friedreich’s ataxia is a rare hereditary neurodegenerative disease caused by decreased levels of the mitochondrial protein frataxin. Similar to other neurodegenerative pathologies, previous studies suggested that astrocytes might contribute to the progression of the disease. To fully understand the mechanisms underlying neurodegeneration in Friedreich’s ataxia, we investigated the reactivity status and functioning of cultured human astrocytes after frataxin depletion using an RNA interference-based approach and tested the effect of pharmacologically modulating the SHH pathway as a novel neuroprotective strategy. Results We observed loss of cell viability, mitochondrial alterations, increased autophagy and lipid accumulation in cultured astrocytes upon frataxin depletion. Besides, frataxin-deficient cells show higher expression of several A1-reactivity markers and release of pro-inflammatory cytokines. Interestingly, most of these defects were prevented by chronically treating the cells with the smoothened agonist SAG. Furthermore, in vitro culture of neurons with conditioned medium from frataxin-deficient astrocytes results in a reduction of neuronal survival, neurite length and synapse formation. However, when frataxin-deficient astrocytes were chronically treated with SAG, we did not observe these alterations in neurons. Conclusions Our results demonstrate that the pharmacological activation of the SHH pathway could be used as a target to modulate astrocyte reactivity and neuron–glia interactions to prevent neurodegeneration in Friedreich’s ataxia.

Neuroinflammation in Friedreich’s Ataxia

“…In contrast, the mice in which FXN was knocked out in astrocytes later in life did not give rise to apparent neurological phenotypes, indicating that developing cerebellar astrocytes are more vulnerable to the lack of FXN, and suggesting a role of astrocytes in the progression of the disease [ 43 ]. Extensive neuroinflammation has been observed in YG8R mice, where FXN loss induced increased satellite cell proliferation, extensive astrocytosis, and an influx of inflammatory OX42 (CD11b/c)-positive cells in both the spinal cord and cerebellar dentate nucleus [ 42 ]. Consistently, in the KIKO mouse model, a substantial increase in astrocytosis was detected following LPS injection in the cerebellum of FRDA compared with non-transgenic mice, suggesting an increased vulnerability to inflammation, as observed for the microglia [ 40 ].…”

“…The results indicate that attenuating neuroinflammation slows down the progression of the disease. The neuroprotective action of this combined treatment was indeed exerted at a clinical level, resulting in a significant improvement in motor coordination and locomotor activity, even after the onset of neurological symptoms [ 42 ].…”

Frataxin-deficient human brain microvascular endothelial cells lose polymerized actin and are paracellularly permeable –implications for blood-brain barrier integrity in Friedreich’s Ataxia

“…Molecularly, the inflammatory cytokine IL-6 is upregulated in FRDA circulation, a pathway which in neurodegeneration is linked to decreased protein expression of occludin (TJ protein) and cadherin (adherens junctions), causing BBB breakdown. IL-6 also activates the M1 inflammatory microglial phenotype, which is observed in FRDA, and both contribute to BBB-breakdown and hyperpermeability in other models of neurodegeneration [56][57][58] . Further compounding evidence for brain iron accumulation via paracellular rather than transcellular flux is indirect downregulation by IL-6 of the iron exporter ferroportin [59][60][61][62][63] .…”

Perspectives on current models of Friedreich’s ataxia

“…Additionally, there are many studies addressing potentially promising therapeutic approaches for FRDA pathophysiology. For instance, smoothened agonist (SAG) treatment in FXN knockdown human astrocytes resulted in increased neuron viability, neurite length, and synapse formation when these cells were injected into the mice brain ( Vicente-Acosta et al, 2022 ). Additionally, Cur@SF, which are nanospheres containing curcumin in silk fibroins, reduced the levels of ROS in FRDA patient-derived fibroblasts and reduced FRDA phenotypes in YG8R mice models ( Xu et al, 2022 ).…”

“…In vivo models of FRDA include yeast, nematode worm, fruit fly, and mice (Babcock et al, 1997;Radisky et al, 1999;Puccio et al, 2001;Al-Mahdawi et al, 2004;Vázquez-Manrique et al, 2006;Virmouni et al, 2014;Monnier et al, 2018), whereas FRDA in vitro models include patient-derived cells such as immortalized lymphoblastoid cells, primary fibroblasts (Li et al, 2016;Agro and Diaz-Nido, 2020;Misiorek et al, 2020;Johnson et al, 2021), FRDA-derived iPSCs (Angulo et al, 2021;Kelekci et al, 2021), and iPSC-based models such as neurons, cardiomyocytes, and beta cells (Crombie et al, 2016;Schreiber et al, 2019) (Figure 1). All these FRDA models need to imitate the symptoms of FRDA patients.…”