Caloric restriction triggers morphofunctional remodeling of astrocytes and enhances synaptic plasticity in the mouse hippocampus

Abstract

Calorie-restricted (CR) diet has multiple beneficial effects on brain function. Here we report morphological and functional changes in hippocampal astrocytes in 3-months-old mice subjected to 1 month of the diet. Whole-cell patch-clamp recordings were performed in the CA1 stratum (str.) radiatum astrocytes of hippocampal slices. The cells were also loaded with fluorescent dye through the patch pipette. CR did not affect the number of astrocytic branches but increased the volume fraction (VF) of distal perisynaptic astrocytic leaflets. The astrocyte growth did not lead to a decrease in the cell input resistance, which may be attributed to a decrease in astrocyte coupling through the gap junctions. Western blotting revealed a decrease in the expression of Cx43 but not Cx30. Immunocytochemical analysis demonstrated a decrease in the density and size of Cx43 clusters. Cx30 cluster density did not change, while their size increased in the vicinity of astrocytic soma. CR shortened K + and glutamate transporter currents in astrocytes in response to 5 × 50 Hz Schaffer collateral stimulation. However, no change in the expression of astrocytic glutamate transporter 1 (GLT-1) was observed, while the level of glutamine synthetase (GS) decreased. These findings suggest that enhanced enwrapping of synapses by the astrocytic leaflets reduces glutamate and K + spillover. Reduced spillover led to a decreased contribution of extrasynaptic N2B containing N-methyl-D-aspartate receptors (NMDARs) to the tail of burst-induced EPSCs. The magnitude of long-term potentiation (LTP) in the glutamatergic CA3-CA1 synapses was significantly enhanced after CR. This enhancement was abolished by N2B-NMDARs antagonist. Our findings suggest that astrocytic morphofunctional remodeling is responsible for enhanced synaptic plasticity, which provides a basis for improved learning and memory reported after CR.

Astrocytes dystrophy in ageing brain parallels impaired synaptic plasticity

“…However, a mass of tiny peripheral astrocytic processes, the leaflets, cannot be resolved with diffraction-limited optical methods (Gavrilov et al, 2018). To quantify age-related changes in peripheral and perisynaptic processes, we analysed their VF as previously described (Medvedev et al, 2014;Popov et al, 2020;Wu et al, 2019). The VF of optically unresolved processes was estimated as a ratio of fluorescence along the astrocytic anatomic domain cross-section to the maximal fluorescence of soma ( Fig.…”

“…Astrocyte morphological remodelling can affect various aspects of astrocyte physiology such as membrane ionic permeability, potassium and glutamate clearance and Ca 2+ dynamics (Plata et al, 2018;Popov et al, 2020). Although astrocytes are electrically non-excitable cells and do not generate action potentials, they are not electrically idle.…”

“…Decreased VF of fine astrocyte processes may associate with reduced astrocytic coverage of synapses, which, in consequence, may affect glutamate and K + clearance (Oliet et al, 2001;Plata et al, 2018;Popov et al, 2020). To test this hypothesis, we monitored glutamate transporter and K + uptake mediated currents (IGluT and IK, respectively) in voltage-clamped CA1 str.radiatum astrocytes in response to stimulation of Schaffer collaterals.…”

“…An image containing astrocyte soma was selected in the z-stack. Special attention was paid that the fluorescence of soma was not saturated (Popov et al, 2020;Wu et al, 2019). Five cross-sections for each cell were plotted via the centre of soma at the angle of 72˚ from each other.…”

“…Astrocytic perisynaptic processes are dynamic structures, which by changing their morphology affect the degree of synaptic coverage thus influencing synaptic transmission. Such morphological plasticity of astrocytes, in particular, occurs upon systemic states (such as lactation (Oliet et al, 2001)) or in response to environmental stimulation (such as dietary changes (Popov et al, 2020)). Classical analysis of astrocytic morphology based on GFAP immunocytochemistry does not access perisynaptic structures because of their small size and absence of GFAP filaments.…”