Alexander Disease-Associated Glial Fibrillary Acidic Protein Mutations in Mice Induce Rosenthal Fiber Formation and a White Matter Stress Response

Abstract

Mutations in the gene for the astrocyte specific intermediate filament, glial fibrillary acidic protein (GFAP), cause the rare leukodystrophy Alexander disease (AxD). To study the pathology of this primary astrocyte defect, we have generated knock-in mice with missense mutations homologous to those found in humans. In this report, we show that mice with GFAP-R76H and -R236H mutations develop Rosenthal fibers, the hallmark protein aggregates observed in astrocytes in AxD, in the hippocampus, corpus callosum, olfactory bulbs, subpial, and periventricular regions. Astrocytes in these areas appear reactive and total GFAP expression is elevated. Although general white matter architecture and myelination appear normal, when crossed with an antioxidant response element reporter line, the mutant mice show a distinct pattern of reporter-gene induction that is especially prominent in the corpus callosum, and histochemical staining reveals accumulation of iron in the same region. The mutant mice have a normal lifespan and show no overt behavioral defects, but are more susceptible to kainate-induced seizures. Although these mice demonstrate increased GFAP expression by themselves, further elevation of GFAP via crosses to GFAP transgenic animals leads to a shift in GFAP solubility, an increased stress response, and ultimately death. The mice do not display the full spectrum of pathology observed in human infantile AxD, but may more closely resemble the adult form of the disease. These studies provide formal proof linking GFAP mutations with Rosenthal fibers and oxidative stress, and correlate gliosis and GFAP protein levels to the severity of the disease.

Emerging role of autophagy in pediatric neurodegenerative and neurometabolic diseases

“…66 Using different mouse models, the accumulation of RFs was shown to be caused by mutant GFAP protein and also by overexpression of wild-type human GFAP, suggesting that elevation of total levels of GFAP is a critical element. 67,68 All GFAP mutations discovered in AxD patients allow production of functional full-length mutant protein, 69 but mutant GFAP tends to form abnormally large, soluble oligomers, 70 and hence, progressive accumulation of mutant GFAP impacts protein metabolism in several ways. Astrocytes and RFs in AxD contain abundant amounts of chaperone proteins suggesting cell stress potentially through the accumulation of misfolded GFAP.…”

Intermediate Filaments in Neurodegenerative Diseases

“…The discovery of GFAP mutations led to the generation of knock-in mice with missense mutations homologous to those found in humans (R76H and R236H, which correspond to the R79H and R239H mutations in human) [165,166]. If the presence of mutant GFAP per se seemed insufficient for aggregate formation, a 30% increase in GFAP content over that in wild-type induced the formation of Rosenthal fibers in multiples sites throughout the CNS [166].…”

“…This indicates that the presence of GFAP aggregates containing mutant GFAP is not sufficient to induce a major phenotype of AXD, even though it causes some abnormalities in the mouse. Interestingly, further elevation of GFAP via crosses to GFAP transgenic animals led to a shift in GFAP solubility, an increased stress response, and ultimately death [165]. This correlates GFAP protein levels to the severity of the disease.…”

“…It thus appeared that accumulations of GFAP protein would be more deleterious to the astrocytes than the mutant protein itself. However, as a positive consequence, up-regulation of αB-crystallin and HSP27 were also associated to the aggregation of GFAP in AXD patients [153,170] as well as in cell and animal models [159,165,168]. Increased αB-crystallin levels would contribute to the disaggregation of GFAP aggregates and could protect cells from apoptotic events [171].…”

The Myelin Mutants as Models to Study Myelin Repair in the Leukodystrophies

“…The infantile form of Alexander disease has profound frontal lobe myelin pathology [1]. The disease has been found to result from mutations of the glial fibrillary acidic protein gene [33], but attempts to create an animal model using knockout or knockin strategies have failed to produce any myelin changes [33]. The final disease to be considered is Canavan disease, which results from mutations in the aspartoacylase gene, leading to profound vacuolation of white matter as is found in a knockout of the gene [34,35] and in a point mutation in the rat model [36].…”