Megalencephalic leukoencephalopathy with subcortical cysts (MLC), a rare leukodystrophy characterized by macrocephaly, subcortical fluid cysts and myelin vacuolation, has been linked to mutations in the MLC1 gene. This gene encodes a membrane protein that is highly expressed in astrocytes. Based on MLC pathological features, it was proposed that astrocyte-mediated defects in ion and fluid homeostasis could account for the alterations observed in MLC-affected brains. However, the role of MLC1 and the effects of pathological mutations on astrocyte osmoregulatory functions have still to be demonstrated. Using human astrocytoma cells stably overexpressing wild-type MLC1 or three known MLC-associated pathological mutations, we investigated MLC1 involvement in astrocyte reaction to osmotic changes using biochemical, dynamic video imaging and immunofluorescence techniques. We have found that MLC1 overexpressed in astrocytoma cells is mainly localized in the plasma membrane, is part of the Na,K-ATPase-associated molecular complex that includes the potassium channel Kir4.1, syntrophin and aquaporin-4 and functionally interacts with the calcium permeable channel TRPV4 (transient receptor potential vanilloid-4 cation channel) which mediates swelling-induced cytosolic calcium increase and volume recovery in response to hyposmosis. Pathological MLC mutations cause changes in MLC1 expression and intracellular localization as well as in the astrocyte response to osmotic changes by altering MLC1 molecular interactions with the Na,K-ATPase molecular complex and abolishing the increase in calcium influx induced by hyposmosis and treatment with the TRPV4 agonist 4αPDD. These data demonstrate, for the first time, that MLC1 plays a role in astrocyte osmo-homeostasis and that defects in intracellular calcium dynamics may contribute to MLC pathogenesis.

Regulatory T (Treg) cells are critical in inducing and maintaining tolerance. Despite progress in understanding the basis of immune tolerance, mechanisms and molecules involved in the generation of Treg cells remain poorly understood. IFN regulatory factor (IRF)-1 is a pleiotropic transcription factor implicated in the regulation of various immune processes. In this study, we report that IRF-1 negatively regulates CD4+CD25+ Treg cell development and function by specifically repressing Foxp3 expression. IRF-1-deficient (IRF-1−/−) mice showed a selective and marked increase of highly activated and differentiated CD4+CD25+Foxp3+ Treg cells in thymus and in all peripheral lymphoid organs. Furthermore, IRF-1−/− CD4+CD25− T cells showed extremely high bent to differentiate into CD4+CD25+Foxp3+ Treg cells, whereas restoring IRF-1 expression in IRF-1−/− CD4+CD25− T cells impaired their differentiation into CD25+Foxp3+ cells. Functionally, both isolated and TGF-β-induced CD4+CD25+ Treg cells from IRF-1−/− mice exhibited more increased suppressive activity than wild-type Treg cells. Such phenotype and functional characteristics were explained at a mechanistic level by the finding that IRF-1 binds a highly conserved IRF consensus element sequence (IRF-E) in the foxp3 gene promoter in vivo and negatively regulates its transcriptional activity. We conclude that IRF-1 is a key negative regulator of CD4+CD25+ Treg cells through direct repression of Foxp3 expression.

Megalencephalic leucoencephalopathy with subcortical cysts (MLC) is a rare congenital leucodystrophy caused by mutations in MLC1, a membrane protein of unknown function. MLC1 expression in astrocyte end-feet contacting blood vessels and meninges, along with brain swelling, fluid cysts and myelin vacuolation observed in MLC patients, suggests a possible role for MLC1 in the regulation of fluid and ion homeostasis and cellular volume changes. To identify MLC1 direct interactors and dissect the molecular pathways in which MLC1 is involved, we used NH2-MLC1 domain as a bait to screen a human brain library in a yeast two-hybrid assay. We identified the β1 subunit of the Na,K-ATPase pump as one of the interacting clones and confirmed it by pull-downs, co-fractionation assays and immunofluorescence stainings in human and rat astrocytes in vitro and in brain tissue. By performing ouabain-affinity chromatography on astrocyte and brain extracts, we isolated MLC1 and the whole Na,K-ATPase enzyme in a multiprotein complex that included Kir4.1, syntrophin and dystrobrevin. Because Na,K-ATPase is involved in intracellular osmotic control and volume regulation, we investigated the effect of hypo-osmotic stress on MLC1/Na,K-ATPase relationship in astrocytes. We found that hypo-osmotic conditions increased MLC1 membrane expression and favoured MLC1/Na,K-ATPase-β1 association. Moreover, hypo-osmosis induced astrocyte swelling and the reversible formation of endosome-derived vacuoles, where the two proteins co-localized. These data suggest that through its interaction with Na,K-ATPase, MLC1 is involved in the control of intracellular osmotic conditions and volume regulation in astrocytes, opening new perspectives for understanding the pathological mechanisms of MLC disease.

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IntroductionIn the middle of the nineteenth century Rudolf ASTROCYTES: EMERGING STARS IN LEUKODYSTROPHY PATHOGENESIS AbstractAstrocytes are the predominant glial cell population in the central nervous system (CNS). Once considered only passive scaffolding elements, astrocytes are now recognised as cells playing essential roles in CNS development and function. They control extracellular water and ion homeostasis, provide substrates for energy metabolism, and regulate neurogenesis, myelination and synaptic transmission. Due to these multiple activities astrocytes have been implicated in almost all brain pathologies, contributing to various aspects of disease initiation, progression and resolution. Evidence is emerging that astrocyte dysfunction can be the direct cause of neurodegeneration, as shown in Alexander's disease where myelin degeneration is caused by mutations in the gene encoding the astrocyte-specific cytoskeleton protein glial fibrillary acidic protein. Recent studies point to a primary role for astrocytes in the pathogenesis of other genetic leukodystrophies such as megalencephalic leukoencephalopathy with subcortical cysts and vanishing white matter disease. The aim of this review is to summarize current knowledge of the pathophysiological role of astrocytes focusing on their contribution to the development of the above mentioned leukodystrophies and on new perspectives for the treatment of neurological disorders. provide some examples of neurological diseases whose pathogenic mechanisms directly alter astrocyte physiology. We will then discuss in more detail the genetic leukodystrophies in which a primary pathogenic role for astrocytes has been described and the possibility to target these cells for therapeutic intervention. Keywords Astrocytes in CNS physiologyEvidence accumulated over the last 20 years has revealed that astrocytes are mainly devoted to (Table 1 Astrocyte dysfunction as primary cause of brain diseases: focus on cystic leukodystrophies (Table 3) The first example of a disease with a primary astrocyte defect is Alexander's disease ( Astrocytes in brain pathology Alexander's disease (AxD)AxD ( While these studies suggest that high levels of wild-type GFAP can reproduce the astrocyte phenotype of AxD, studies performed in cultured astrocytes showed that mutant GFAP protein accumulates more rapidly and at higher levels than the wild-type protein [151,152].Thus, the precise mechanism through which the diffuse swelling aspect of the abnormal hyperintense signal in T2 weighted image also involving the internal capsule; (B) the same areas appear markedly hypointense in the FLAIR weighetd images indicating abnormal vacuolation of the white matter; (C) the abnormal hyperintense T2 weighted signal also involves the white matter of the cerebellum; (D) the diffuse swelling aspect with increased vacuolation is also clear in the parasaggital T1 weighted image. A B C D