Liver X receptor-agonist treatment rescues degeneration in a Drosophila model of hereditary spastic paraplegia

Abstract

Hereditary spastic paraplegias (HSPs) are a group of inherited, progressive neurodegenerative conditions characterised by prominent lower-limb spasticity and weakness, caused by a length-dependent degeneration of the longest corticospinal upper motor neurons. While more than 80 spastic paraplegia genes (SPGs) have been identified, many cases arise from mutations in genes encoding proteins which generate and maintain tubular endoplasmic reticulum (ER) membrane organisation. The ER-shaping proteins are essential for the health and survival of long motor neurons, however the mechanisms by which mutations in these genes cause the axonopathy observed in HSP have not been elucidated. To further develop our understanding of the ER-shaping proteins, this study outlines the generation of novel in vivo and in vitro models, using CRISPR/Cas9-mediated gene editing to knockout the ER-shaping protein ADP-ribosylation factor-like 6 interacting protein 1 (ARL6IP1), mutations in which give rise to the HSP subtype SPG61. Loss of Arl6IP1 in Drosophila results in progressive locomotor deficits, emulating a key aspect of HSP in patients. ARL6IP1 interacts with ER-shaping proteins and is required for regulating the organisation of ER tubules, particularly within long motor neuron axons. Unexpectedly, we identified physical and functional interactions between ARL6IP1 and the phospholipid transporter oxysterol-binding protein-related protein 8 in both human and Drosophila model systems, pointing to a conserved role for ARL6IP1 in lipid homeostasis. Furthermore, loss of Arl6IP1 from Drosophila neurons results in a cell non-autonomous accumulation of lipid droplets in axonal glia. Importantly, treatment with lipid regulating liver X receptor-agonists blocked lipid droplet accumulation, restored axonal ER organisation, and improved locomotor function in Arl6IP1 knockout Drosophila. Our findings indicate that disrupted lipid homeostasis contributes to neurodegeneration in HSP, identifying a potential novel therapeutic avenue for the treatment of this disorder.

An Automated Imaging-Based Screen for Genetic Modulators of ER Organisation in Cultured Human Cells

“…Numerous subtypes of HSP are caused by mutations encoding ER-resident enzymes involved in lipid synthesis, e.g., the phospholipase PN-PLA6 (SPG39) [9,10], LD production, e.g., Seipin [11,12], and LD turnover, e.g., Spartin (SPG20) [13,14]. LD defects have also been reported in cell and animal models of HSP with mutations in ER-shaping proteins [15,16]. Another common feature of HSP-causing mutations and model systems is the regulation of mitochondrial function and organisation.…”

Liver X Receptor Regulation of Glial Cell Functions in the CNS

“…LXRα is mainly expressed in organs involved in lipid metabolism, such as the liver, intestine, adipose tissue, and macrophages. LXRβ is not ubiquitously expressed as has been reported, but it does have a wider tissue distribution being expressed in the immune system, glial cells in the CNS, gall bladder, islets of the pancreas, and prostate epithelium [ 8 , 9 , 10 , 11 , 12 , 13 ].…”

Identification of Novel Compounds Targeting the Liver X Receptor (LXR): in-silico studies, screening, molecular docking, and chemico-pharmacokinetic analysis

“…An agonist of pan LXR-α/β, GSK3987 has EC50s of 40-50 nM reduces triglyceride accumulation and enhances cellular cholesterol e ux in mouse models [24]. Several LXR agonists tested in clinical trials for metabolic diseases including LXR-623, BMS-779788, BMS-852927, and AZ876 [18, [25][26][27][28][29][30]. Despite LXR agonist's e cacy, there continue to be several unmet needs in its development and application.…”