Poor cerebral inflammatory response in eIF2B knock-in mice: implications for the aetiology of vanishing white matter disease.

BACKGROUND: Mutations in any of the five subunits of eukaryotic translation initiation factor 2B (eIF2B) can lead to an inherited chronic-progressive fatal brain disease of unknown aetiology termed leucoencephalopathy with vanishing white matter (VWM). VWM is one of the most prevalent childhood white matter disorders, which markedly deteriorates after inflammation or exposure to other stressors. eIF2B is a major housekeeping complex that governs the rate of global protein synthesis under normal and stress conditions. A previous study demonstrated that Eif2b5(R132H/R132H) mice suffer delayed white matter development and fail to recover from cuprizone-induced demyelination, although eIF2B enzymatic activity in the mutant brain is reduced by merely 20%. PRINCIPAL FINDINGS: Poor astrogliosis was observed in Eif2b5(R132H/R132H) mice brain in response to systemic stress induced by peripheral injections of lipopolysaccharide (LPS). Even with normal rates of protein synthesis under normal conditions, primary astrocytes and microglia isolated from mutant brains fail to adequately synthesise and secrete cytokines in response to LPS treatment despite proper induction of cytokine mRNAs. CONCLUSIONS: The mild reduction in eIF2B activity prevents the appropriate increase in translation rates upon exposure to the inflammatory stressor LPS. The data underscore the importance of fully-functional translation machinery for efficient cerebral inflammatory response upon insults. It highlights the magnitude of proficient translation rates in restoration of brain homeostasis via microglia-astrocyte crosstalk. This study is the first to suggest the involvement of microglia in the pathology of VWM disease. Importantly, it rationalises the deterioration of clinical symptoms upon exposure of VWM patients to physiological stressors and provides possible explanation for their high phenotypic variability.

A point mutation in translation initiation factor eIF2B leads to function--and time-specific changes in brain gene expression.

BACKGROUND: Mutations in eukaryotic translation initiation factor 2B (eIF2B) cause Childhood Ataxia with CNS Hypomyelination (CACH), also known as Vanishing White Matter disease (VWM), which is associated with a clinical pathology of brain myelin loss upon physiological stress. eIF2B is the guanine nucleotide exchange factor (GEF) of eIF2, which delivers the initiator tRNA(Met) to the ribosome. We recently reported that a R132H mutation in the catalytic subunit of this GEF, causing a 20% reduction in its activity, leads under normal conditions to delayed brain development in a mouse model for CACH/VWM. To further explore the effect of the mutation on global gene expression in the brain, we conducted a wide-scale transcriptome analysis of the first three critical postnatal weeks. METHODOLOGY/PRINCIPAL FINDINGS: Genome-wide mRNA expression of wild-type and mutant mice was profiled at postnatal (P) days 1, 18 and 21 to reflect the early proliferative stage prior to white matter establishment (P1) and the peak of oligodendrocye differentiation and myelin synthesis (P18 and P21). At each developmental stage, between 441 and 818 genes were differentially expressed in the mutant brain with minimal overlap, generating unique time point-specific gene expression signatures. CONCLUSIONS: The current study demonstrates that a point mutation in eIF2B, a key translation initiation factor, has a massive effect on global gene expression in the brain. The overall changes in expression patterns reflect multiple layers of indirect effects that accumulate as the brain develops and matures. The differentially expressed genes seem to reflect delayed waves of gene expression as well as an adaptation process to cope with hypersensitivity to cellular stress.

Species-specific microRNA roles elucidated following astrocyte activation.

MicroRNAs (miRNAs) are short non-coding RNAs that play a central role in regulation of gene expression by binding to target genes. Many miRNAs were associated with the function of the central nervous system (CNS) in health and disease. Astrocytes are the CNS most abundant glia cells, providing support by maintaining homeostasis and by regulating neuronal signaling, survival and synaptic plasticity. Astrocytes play a key role in repair of brain insults, as part of local immune reactivity triggered by inflammatory or pathological conditions. Thus, astrocyte activation, or astrogliosis, is an important outcome of the innate immune response, which can be elicited by endotoxins such as lipopolysaccharide (LPS) and cytokines such as interferon-gamma (IFN-γ). The involvement of miRNAs in inflammation and stress led us to hypothesize that astrogliosis is mediated by miRNA function. In this study, we compared the miRNA regulatory layer expressed in primary cultured astrocyte derived from rodents (mice) and primates (marmosets) brains upon exposure to LPS and IFN-γ. We identified subsets of differentially expressed miRNAs some of which are shared with other immunological related systems while others, surprisingly, are mouse and rat specific. Of interest, these specific miRNAs regulate genes involved in the tumor necrosis factor-alpha (TNF-α) signaling pathway, indicating a miRNA-based species-specific regulation. Our data suggests that miRNA function is more significant in the mechanisms governing astrocyte activation in rodents compared to primates.

A mouse model for eukaryotic translation initiation factor 2B-leucodystrophy reveals abnormal development of brain white matter.

Eukaryotic translation initiation factor 2B is a major housekeeping complex that governs the rate of global protein synthesis under normal and stress conditions. Mutations in any of its five subunits lead to leucoencephalopathy with vanishing white matter, an inherited chronic-progressive fatal brain disease with unknown aetiology, which is among the most prevalent childhood white matter disorders. We generated the first animal model for the disease by introducing a point mutation into the mouse Eif2b5 gene locus, leading to R132H replacement corresponding to the clinically significant human R136H mutation in the catalytic subunit. In contrast to human patients, mice homozygous for the mutant Eif2b5 allele (Eif2b5(R132H/R132H) mice) enable multiple analyses under a defined genetic background during the pre-symptomatic stages and during recovery from a defined brain insult. Time-course magnetic resonance imaging revealed for the first time the delayed development of the brain white matter due to the mutation. Electron microscopy demonstrated a higher proportion of small-calibre nerve fibres. Immunohistochemistry detected an abnormal abundance of oligodendrocytes and astrocytes in the brain of younger animals, as well as an abnormal level of major myelin proteins. Most importantly, mutant mice failed to recover from cuprizone-induced demyelination, reflecting an increased sensitivity to brain insults. The anomalous development of white matter in Eif2b5(R132H/R132H) mice underscores the importance of tight translational control to normal myelin formation and maintenance.