Studies on the phenotype of LXRβ-/- mice : from malabsorption to amyotrophic lateral sclerosis

Abstract

Liver X Receptors, LXRalpha and LXRbeta (NR1H2) are nuclear receptors belonging to the superfamily of ligand-activated transcription factors with a key role in the control of lipid and glucose metabolism. Studies of the phenotype of LXRalpha-/-mice and LXRbeta-/-mice have shown that these nuclear receptors have specific and distinct roles, although they share very high sequence homology (78%). In particular, unlike LXRalpha-/-mice, LXRbeta-/-mice are resistant to obesity when fed with a high fat diet and have a decreased gonadal fat pad during standard diet feeding. In addition, they develop a motor neuron disease that resembles amyotrophic lateral sclerosis by the age of 7 months. The studies in this thesis are aimed at investigating the mechanisms through which loss of LXRbeta leads to disease.

The first study investigated the pathogenesis of motor neuron disease in LXRbeta-/- mice. It has been shown that in the spinal cord of patients with amyotrophic lateral sclerosis, there is an accumulation of sphingomyelin, ceramides and cholesterol esters: phytosterols are known ligands of LXRs and are secreted into the intestinal lumen by ABCG transporters. These transporters are LXR-regulated genes. In the first study we treated LXRbeta-/- mice with beta-sitosterol, a known motoneuron toxin thought to be involved in ALS. After 3 weeks of treatment, in LXRbeta-/- mice but not in wild type littermates, there was a reduction in the number of motor neurons in the lumbar region of the spinal cord and in the surviving motor neurons there were aggregates of ubiquitin and TDP-43. In addition, in the substantia nigra there was a loss of tyrosine hydroxylasepositive dopaminergic neurons and an increase in the number of activated microglia. Brain cholesterol concentrations were higher in LXRbeta-/- than in their wild type counterparts, and treatment with beta-sitosterol reduced brain cholesterol in both WT and LXRbeta-/- mice. Levels of the intestinal transporters, ABCG5/8 and Niemann-Pick C1 Like 1, were not affected by the loss of LXRbeta and/or treatment with beta-sitosterol. In conclusion, these data indicate that multiple mechanisms are involved in the sensitivity of LXRbeta-/- mice to beta-sitosterol: activation of microglia, accumulation of protein aggregates in the cytoplasm of large motor neurons, and depletion of brain cholesterol.

The second paper examined the resistance to gain weight of LXRbeta-/- mice. We demonstrated that these mice at 11 months of age on a standard diet, exhibit a pancreatic exocrine insufficiency that could explain their resistance to gain weight. Moreover, we showed a marked reduction in the expression of the water channel aquaporin-1 in pancreatic ducts of LXRbeta-/-mice. This reduction is likely responsible for the thick and dense secretion visualized in electron microscopy and for the pancreatic insufficiency. With a specific antibody we demonstrated the presence of LXRbeta in the nuclei of epithelial cells in pancreatic ducts and by treatment of wild type mice with an LXR agonist we could demonstrate an increase in the level of mRNA of aquaporin-1. These findings suggest that LXRbeta plays an important role in controlling pancreatic juice secretion through the regulation of water transport.

Interestingly, patients affected by ALS also exhibit exocrine insufficiency. Thus the two major characteristics of LXRbeta-/- mice may be part of a single syndrome whose common etiology stems from defective LXRbeta signaling.