Metabolic Enzyme Alterations and Astrocyte Dysfunction in a Murine Model of Alexander Disease With Severe Reactive Gliosis

Abstract

Alexander disease (AxD) is a rare and fatal neurodegenerative disorder caused by mutations in the gene encoding glial fibrillary acidic protein (GFAP). In this report, a mouse model of AxD ( GFAP Tg ; Gfap +/ R236H ) was analyzed that contains a heterozygous R236H point mutation in murine Gfap as well as a transgene with a GFAP promoter to overexpress human GFAP. Using label-free quantitative proteomic comparisons of brain tissue from GFAP Tg ; Gfap +/ R236H versus wild-type mice confirmed upregulation of the glutathione metabolism pathway and indicated proteins were elevated in the peroxisome proliferator-activated receptor (PPAR) signaling pathway, which had not been reported previously in AxD. Relative protein-level differences were confirmed by a targeted proteomics assay, including proteins related to astrocytes and oligodendrocytes. Of particular interest was the decreased level of the oligodendrocyte protein, 2-hydroxyacylsphingosine 1-beta-galactosyltransferase (Ugt8), since Ugt8 -deficient mice exhibit a phenotype similar to GFAP Tg ; Gfap +/ R236H mice ( e.g. , tremors, ataxia, hind-limb paralysis). In addition, decreased levels of myelin-associated proteins were found in the GFAP Tg ; Gfap +/ R236H mice, consistent with the role of Ugt8 in myelin synthesis. Fabp7 upregulation in GFAP Tg ; Gfap +/ R236H mice was also selected for further investigation due to its uncharacterized association to AxD, critical function in astrocyte proliferation, and functional ability to inhibit the anti-inflammatory PPAR signaling pathway in models of amyotrophic lateral sclerosis (ALS). Within Gfap + astrocytes, Fabp7 was markedly increased in the hippocampus, a brain region subjected to extensive pathology and chronic reactive gliosis in GFAP Tg ; Gfap +/ R236H mice. Last, to determine whether the findings in GFAP Tg ; Gfap +/ R236H mice are present in the human condition, AxD patient and control samples were analyzed by Western blot, which indicated that Type I AxD patients have a significant fourfold upregulation of FABP7. However, immunohistochemistry analysis showed that UGT8 accumulates in AxD patient subpial brain regions where abundant amounts of Rosenthal fibers are located, which was not observed in the GFAP Tg ...

Frataxin deficiency in proprioceptive neurons is causal to inflammatory and glial responses in dorsal root ganglia

“…Their downregulation suggests altered myelogenesis and/or defective myelin repair, which is consistent with the previous observation that at 3.5 weeks of age, sciatic nerves of Pv-cKO-FXN mice already displayed signs of myelin inner tongue abnormalities and beginning of neuropathy 14 . Furthermore, abnormal expression of Pmp22 or Utg8a produces neurodegenerative phenotypes resembling Charcot-Marie-Tooth or associated to Alexander's diseases 88,92 .…”

Human iPSC-Derived Astrocytes: A Powerful Tool to Study Primary Astrocyte Dysfunction in the Pathogenesis of Rare Leukodystrophies

“…In addition, a mouse model carrying a heterozygous R236H GFAP point mutation as well as a transgene with a GFAP promoter to overexpress human GFAP, showed a decrease in the expression of UDP-galactose-ceramide galactosyltransferase (Ugt8), a regulator of myelin membrane synthesis. Accordingly, these animals also expressed lower levels of the basilar myelin protein constituents (Cnp, Mbp, Mog, Mobp, Mag, and Plp1) [ 79 ] suggesting this model as a useful tool to study AxD myelination defects. Newly generated zebrafish models revealed instead a useful system to study the early stages of disease [ 80 , 81 ], but, overall, these animal models ( Figure 2 ) did not succeed in clarifying the relationship between diseased astrocytes and oligodendrocyte/myelin defects.…”

Alexander disease GFAP R239C mutant shows increased susceptibility to lipoxidation and elicits mitochondrial dysfunction and oxidative stress

“…There are indirect pieces of evidence from AxD models suggesting the occurrence of oxidative stress. An upregulation of Nrf2 and several of its targets, including enzymes involved in glutathione metabolism, has been observed in mouse models of the disease [ 32 , 33 ]. In these models, both overexpression and knockout of Nrf2 decreases the levels of GFAP [ 34 , 35 ], whereas overexpression of antioxidant enzymes modulates mutant GFAP toxicity in a fly model [ 36 ].…”