Synaptic NMDA receptors (NMDARs) carry inward Ca(2+) current responsible for postsynaptic signaling and plasticity in dendritic spines. Whether the concurrent K(+) efflux through the same receptors into the synaptic cleft has a physiological role is not known. Here, we report that NMDAR-dependent K(+) efflux can provide a retrograde signal in the synapse. In hippocampal CA3-CA1 synapses, the bulk of astrocytic K(+) current triggered by synaptic activity reflected K(+) efflux through local postsynaptic NMDARs. The local extracellular K(+) rise produced by activation of postsynaptic NMDARs boosted action potential-evoked presynaptic Ca(2+) transients and neurotransmitter release from Schaffer collaterals. Our findings indicate that postsynaptic NMDAR-mediated K(+) efflux contributes to use-dependent synaptic facilitation, thus revealing a fundamental form of retrograde synaptic signaling.

Due to the high incidence of nosocomial Candida albicans infection, the first-line drugs for C. albicans infection have been heavily used, and the emergence of drug-resistant strains has gradually increased. Thus, a new antifungal drug or therapeutic method is needed. Chitosan, a product of chitin deacetylation, is considered to be potentially therapeutic for fungal infections because of its excellent biocompatibility, biodegradability and low toxicity. The biocidal action of chitosan against C. albicans shows great commercial potential, but the exact mechanisms underlying its antimicrobial activity are unclear. To reveal these mechanisms, mutant library screening was performed. ADA2 gene, which encodes a histone acetylation coactivator in the SAGA complex, was identified. Transmission electronic microscopy images showed that the surface of chitosan-treated ada2 Δ cells was substantially disrupted and displayed an irregular morphology. Interestingly, the cell wall of ada2 Δ cells was significantly thinner than that of wild-type cells, with a thickness similar to that seen in the chitosan-treated wild-type strain. Although ADA2 is required for chitosan tolerance, expression of ADA2 and several Ada2-mediated cell wall-related genes ( ALS2, PGA45 , and ACE2 ) and efflux transporter genes ( MDR1 and CDR1 ) were significantly inhibited by chitosan. Furthermore, GCN5 encoding a SAGA complex catalytic subunit was inhibited by chitosan, and gcn5 Δ cells exhibited phenotypes comparable to those of ada2 Δ cells in response to chitosan and other cell surface-disrupting agents. This study demonstrated that a potential antifungal mechanism of chitosan against C. albicans operates by inhibiting SAGA complex gene expression, which decreases the protection of the cell surface against chitosan.

Running title: Morphology-dependent calcium activity in astrocytesMain points:• Majority of spontaneous Ca 2+ events start in thin astrocytic processes • Higher surface-to-volume ratio of the process is responsible for larger intracellular Ca 2+ fluctuations • Larger intracellular Ca 2+ fluctuations trigger Ca 2+ -dependent Ca 2+ release Russian Science Foundation . AbstractAstrocytes express a complex repertoire of intracellular Ca 2+ transients (events) that represent a major form of signaling within individual cells and in the astrocytic syncytium. These events have different spatiotemporal profiles, which are modulated by neuronal activity. Spontaneous Ca 2+ events appear more frequently in distal astrocytic processes and independently from each other. However, little is known about the mechanisms underlying such subcellular distribution of the Ca 2+ events. Here we identify the initiation points of the Ca 2+ events within the territory of single astrocytes expressing genetically encoded Ca 2+ indicator GCaMP2 in culture or in hippocampal slices. We found that most of the Ca 2+ events start in thin distal processes. Our mathematical model demonstrated that a high surface-to-volume (SVR) of the thin processes leads to increased amplitude of baseline Ca 2+ fluctuations caused by a stochastic opening of Ca 2+ channels in the plasma membrane. Suprathreshold fluctuations trigger Ca 2+ -induced Ca 2+ release (CICR) from the Ca 2+ stores by activating inositol 1,4,5-trisphosphate (IP 3 ) receptors. In agreement with the model prediction, the spontaneous Ca 2+ events frequency depended on the extracellular Ca 2+ concentration.Astrocytic depolarization by high extracellular K + increased the frequency of the Ca 2+ events through activation of voltage-gated Ca 2+ channels (VGCC) in cultured astrocytes. Our results suggest that the morphological profile of the astrocytic processes is responsible for tuning of the Ca 2+ event frequency. Therefore, the structural plasticity of astrocytic processes can be directly translated into changes in astrocytic Ca 2+ signaling. This may be important for both physiological and pathological astrocyte remodeling.

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Astrocytes express a complex repertoire of intracellular Ca2+ transients (events) that represent a major form of signaling within individual cells and in astrocytic syncytium. These events have different spatiotemporal profiles, which are modulated by neuronal activity. Spontaneous Ca2+ events appear more frequently in distal astrocytic processes and independently from each other. However, little is known about the mechanisms underlying such subcellular distribution of the Ca2+ events. Here, we identify the initiation points of the Ca2+ events within the territory of single astrocytes expressing genetically encoded Ca2+ indicator GCaMP2 in culture or in hippocampal slices. We found that most of the Ca2+ events start in an optimal range of thin distal processes. Our mathematical model demonstrated that a high surface‐to‐volume of the thin processes leads to increased amplitude of baseline Ca2+ fluctuations caused by a stochastic opening of Ca2+ channels in the plasma membrane. Suprathreshold fluctuations trigger Ca2+‐induced Ca2+ release from the Ca2+ stores by activating inositol 1,4,5‐trisphosphate (IP3) receptors. In agreement with the model prediction, the spontaneous Ca2+ events frequency depended on the extracellular Ca2+ concentration. Astrocytic depolarization by high extracellular K+ increased the frequency of the Ca2+ events through activation of voltage‐gated Ca2+ channels in cultured astrocytes. Our results suggest that the morphological profile of the astrocytic processes is responsible for tuning of the Ca2+ events frequency. Therefore, structural plasticity of astrocytic processes can be directly translated into changes in astrocytic Ca2+ signaling. This may be important for both physiological and pathological astrocyte remodeling.

Astrocytes, the main supportive cell type of the brain, show functional impairments upon ageing and in a broad spectrum of neurological disorders. Limited access to human astroglia for pre-clinical studies has been a major bottleneck delaying our understanding of their role in brain health and disease. We demonstrate here that functionally mature human astrocytes can be generated by SOX9 overexpression for 6 days in pluripotent stem cell (PSC)-derived neural progenitor cells. Inducible (i)SOX9-astrocytes display functional properties comparable to primary human astrocytes comprising glutamate uptake, induced calcium responses and cytokine/growth factor secretion. Importantly, electrophysiological properties of iNGN2-neurons co-cultured with iSOX9-astrocytes are indistinguishable from gold-standard murine primary cultures. The high yield, fast timing and the possibility to cryopreserve iSOX9-astrocytes without losing functional properties makes them suitable for scaled-up production for high-throughput analyses. Our findings represent a step forward to an all-human iPSC-derived neural model for drug development in neuroscience and towards the reduction of animal use in biomedical research. Graphical Abstract

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