Astrocytes are homeostatic glial cells in the central nervous system that respond to pathologic changes with a process of activation. Interferon γ (IFNγ), an inflammatory cytokine, induces astrocyte expression of major histocompatibility class II (MHCII) molecules required for antigen presentation. Isolated astrocytes are traditionally cultured with serum; however, serum increases astrocyte basal activation and alters their morphology and intracellular calcium signalling. The impact of both triggers of astrocyte activation on exo-/endocytotic activity in astrocytes is not understood. We have thus studied the properties and dynamics of exo-/endocytotic events in non-treated and IFNγ-treated serum as well as serum-free cultured rat astrocytes. The intracellular distribution of immunofluorescently labelled MHCII molecules was analysed with confocal and structural illumination microscopies. Furthermore, elementary exo-/endocytotic activity was studied by patch clamp membrane capacitance measurements. In IFNγ-treated astrocytes, MHCII molecules predominantly localized to late endo-/lysosomes, but also reached the plasmalemma. Additionally, larger exocytotic vesicles reversibly interacted with the plasmalemma, while full endocytotic events were inhibited. Exo-/endocytotic activity of astrocytes in serum-free culture was unaltered at rest, however their exo-/endocytotic activity in response to ATP stimulation was suppressed. Activated cultured astrocytes instigated by treatment with IFNγ or serum culture supplementation display altered dynamics of elementary exo-/endocytotic activity and prolonged surface retention of MHCII.
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