IFN-γ-induced increase in the mobility of MHC class II compartments in astrocytes depends on intermediate filaments

Abstract

BackgroundIn immune-mediated diseases of the central nervous system, astrocytes exposed to interferon-γ (IFN-γ) can express major histocompatibility complex (MHC) class II molecules and antigens on their surface. MHC class II molecules are thought to be delivered to the cell surface by membrane-bound vesicles. However, the characteristics and dynamics of this vesicular traffic are unclear, particularly in reactive astrocytes, which overexpress intermediate filament (IF) proteins that may affect trafficking. The aim of this study was to determine the mobility of MHC class II vesicles in wild-type (WT) astrocytes and in astrocytes devoid of IFs.MethodsThe identity of MHC class II compartments in WT and IF-deficient astrocytes 48 h after IFN-γ activation was determined immunocytochemically by using confocal microscopy. Time-lapse confocal imaging and Alexa Fluor546-dextran labeling of late endosomes/lysosomes in IFN-γ treated cells was used to characterize the motion of MHC class II vesicles. The mobility of vesicles was analyzed using ParticleTR software.ResultsConfocal imaging of primary cultures of WT and IF-deficient astrocytes revealed IFN-γ induced MHC class II expression in late endosomes/lysosomes, which were specifically labeled with Alexa Fluor546-conjugated dextran. Live imaging revealed faster movement of dextran-positive vesicles in IFN-γ-treated than in untreated astrocytes. Vesicle mobility was lower in IFN-γ-treated IF-deficient astrocytes than in WT astrocytes. Thus, the IFN-γ-induced increase in the mobility of MHC class II compartments is IF-dependent.ConclusionsSince reactivity of astrocytes is a hallmark of many CNS pathologies, it is likely that the up-regulation of IFs under such conditions allows a faster and therefore a more efficient delivery of MHC class II molecules to the cell surface. In vivo, such regulatory mechanisms may enable antigen-presenting reactive astrocytes to respond rapidly and in a controlled manner to CNS inflammation.

Excitable Astrocytes: Ca2+- and cAMP-Regulated Exocytosis

“…Instead they express lysosomal specific markers such as cathepsin D and LAMP1 [38], Rab 7, SNARE protein TI-VAMP/VAMP7, which contributes to tetanus toxin independent exocytosis of ATP [74], and vesicular nucleotide transporter (VNUT) [108], which is involved in ATP storage [109] and release [86] from secretory lysosomes in astrocytes. These vesicles can be specifically labelled with dextrans [39,110], FM dyes, MANT-ATP [38] and exhibit diameters of 300-500 nm [38,78,90] and can co-exist with SLMVs in the same astrocyte [107].…”

“…The appearance of antigen-presenting molecules (MHC class II molecules) in endolysosomal compartments, which then get exposed to the cell surface, takes several hours under conditions when astrocytes are stimulated with inflammatory cytokine interferon-c [39].…”

“…This review addresses the excitability mechanisms involving GPCRs in astrocytes and the vesicular release mechanisms of gliosignals, such as amino acids (glutamate [21,29,30], D-serine [31,32]), peptides (atrial natriuretic peptide (ANP) [33], secretogranin II and chromogranin [34]), nucleotides (adenosine 5 0 -triphosphate (ATP) [35][36][37][38]) and molecules required for facultative antigen presentation by astrocytes, major histocompatibility class II molecules (major histocompatibility complex (MHC) II complexes [39]). First, we briefly discuss the mechanisms of excitability, then we focus on the molecular machinery underlying gliosignal exocytosis by dealing with different vesicle types in astrocytes.…”

Glial fibrillary acidic protein (GFAP) and the astrocyte intermediate filament system in diseases of the central nervous system

“…When the GFAP À/À Vim À/À mice were subjected to focal brain ischemia, they developed larger infarction compared to wild-type mice [73 ]. We are still seeking the mechanistic understanding of some of these phenomena, but a number of studies linked the astrocyte IF system to cell migration [54], viscoelastic properties [74], vesicle trafficking [75][76][77], activation of Erk and c-fos [78], control of glutamate transport and astrocyte gap junctional communication [73 ], reaction to hypoosmotic and oxidative stresses [49,79], reconstruction of blood-brain barrier [80], and interaction with microglia [81,82].…”

Astrocyte Reactivity and Reactive Astrogliosis: Costs and Benefits

“…The same conclusion was made by exposing the GFAP Ϫ/Ϫ Vim Ϫ/Ϫ mice to focal brain ischemia (induced by the transection of the middle cerebral artery), a mouse model of ischemic stroke, which resulted in larger infarction compared with wild-type mice (132). The underlying cellular and molecular mechanisms remain incompletely understood, although the astrocyte IF system has been linked to astrocyte motility (131); viscoelastic properties, which might affect cell migration (139); vesicle trafficking (184,185,241); activation of Erk and c-fos (156); the efficiency of glutamate transport and astrocyte gap junctional communication (132) (135); response to hypoosmotic and oxidative stress and neuroprotective properties (55,58); reconstruction of blood-brain barrier (177); MHC class II molecule presentation (241); and interaction with microglia (125,143). The functions of astrocytes in healthy CNS and in acute and late stages of neurological diseases are exemplified in FIGURE 8C.…”