An iPSC-derived astrocyte model of fragile X syndrome exhibits dysregulated cholesterol homeostasis

Abstract

Cholesterol is an essential membrane structural component and steroid hormone precursor, and is involved in numerous signaling processes. Astrocytes regulate brain cholesterol homeostasis and they supply cholesterol to the needs of neurons. ATP-binding cassette transporter A1 (ABCA1) is the main cholesterol efflux transporter in astrocytes. Here we show dysregulated cholesterol homeostasis in astrocytes generated from human induced pluripotent stem cells (iPSCs) derived from males with fragile X syndrome (FXS), which is the most common cause of inherited intellectual disability. ABCA1 levels are reduced in FXS human and mouse astrocytes when compared with controls. Accumulation of cholesterol associates with increased desmosterol and polyunsaturated phospholipids in the lipidome of FXS mouse astrocytes. Abnormal astrocytic responses to cytokine exposure together with altered anti-inflammatory and cytokine profiles of human FXS astrocyte secretome suggest contribution of inflammatory factors to altered cholesterol homeostasis. Our results demonstrate changes of astrocytic lipid metabolism, which can critically regulate membrane properties and affect cholesterol transport in FXS astrocytes, providing target for therapy in FXS.

Astrocytes in fragile X syndrome

“…The proinflammatory cytokine interleukin 1β (IL-1β) is an activator of human astrocytes leading to production of inflammatory mediators such as cytokines, chemokines, nitric oxide (NO), and reactive oxygen species (ROS) ( Lee et al, 1993 ; Sharma et al, 2007 ) and to down-regulation of glutamate uptake ( Hu et al, 2000 ). IL-1β treatment increases GFAP and chemokine CCL5 mRNA expression more in human FXS iPSC-derived astrocytes than in control astrocytes ( Talvio et al, 2023 ), suggesting that FXS astrocytes are characterized by increased sensitivity to reactivity. This may be reflected in Fmr1 KO mice as impaired brain energy metabolism and increased ROS markers ( el Bekay et al, 2007 ; D’Antoni et al, 2020 ).…”

“…In the transcriptomics analysis of human FXS NPCs, MYD88 was amongst the most significantly overexpressed genes ( Talvio et al, 2022 ) and there are several tentative arguments for MYD88-mediated system overactivity in FXS astrocytes. Expression of MMP-9, the proposed target of minocycline in FXS, is upregulated via a MYD88-dependent mechanism in astrocytes ( Gorina et al, 2011 ), and reduced IL-10 in FXS astrocytes ( Talvio et al, 2023 ) might contribute to increased MYD88 ( Chang et al, 2009 ). TLR4/MyD88-mediated induction of IL-6 is overactive in Fmr1 KO astrocytes ( Krasovska and Doering, 2018 ), implicating involvement of inflammatory factors and suggesting alteration in TLR4/MyD88/PI3K interactions ( Laird et al, 2009 ) in FXS astrocytes.…”

“…Increased lanosterol synthase and lanosterol, an intermediate oxysteroid in the Bloch pathway of cholesterol synthesis, were associated with reduced downstream enzymes and total cholesterol in three out of the four FXS cell lines studied and compared with their isogenic controls. In studies of Fmr1 KO mouse astrocytes, cholesterol, and desmosterol (the last precursor of cholesterol in the Bloch pathway) accumulated in astrocytes ( Talvio et al, 2023 ). Here, cholesterol levels did not differ between human or mouse FMRP-deficient and control astrocyte conditioned medium, suggesting that astrocytes maintained extracellular cholesterol homeostasis to avoid cholesterol toxicity ( Adachi et al, 2022 ; Talvio et al, 2023 ).…”

“…Reduced ABCA1 expression associated with reduced secretion of IL-13 and IL-10, which are implicated in ABCA1 regulation ( Cardilo-Reis et al, 2012 ; Ma et al, 2012 ). Since astrocyte culturing conditions and differentiation protocols for production of iPSC-derived astrocytes can affect lipidomics of astrocytes, they may explain differences in the cholesterol balance observed in different FXS astrocyte models ( Ren et al, 2023 ; Talvio et al, 2023 ). Leukemia inhibitory factor LIF is a factor used to specify astrocytes during differentiation ( Ren et al, 2023 ) and by inducing ABCA1 expression it might modulate cholesterol secretion ( Trouillas et al, 2009 ).…”

“…There are several changes in the lipidome of the Fmr1 KO mouse astrocytes ( Talvio et al, 2023 ). ABCA1 affects membrane structure by transporting cholesterol, sphingomyelin, phosphatidylserine, and phosphatidylcholine (PC) species from the cytoplasmic to the exocytoplasmic leaflet ( Quazi and Molday, 2013 ).…”