Audrey Denizot,
Corrado Calì,
Weiliang Chen,
Iain Hepburn,
Hugues Berry,
Erik De Schutter
Here is the link to the virtual room of the presentation: https://oist.zoom.us/j/97032096119?pwd=ZnRoeWkvNnZGaER6M3dqMzlJZmVBdz09 Password: 429623
2-minute teaser: https://youtu.be/KawxI1RiMrM
Astrocytes, glial cells of the central nervous system, display a striking diversity of Ca2+ signals in response to neuronal activity. 80% of those signals take place in cellular ramifications that are too fine to be resolved by conventional light microscopy [1], often in apposition to synapses (perisynaptic astrocytic processes, PAPs). Understanding Ca2+ signaling in PAPs, where astrocytes potentially regulate neuronal information processing [2], is crucial. At this spatial scale, Ca2+ signals are not distributed uniformly, being preferentially located in so-called Ca2+ hotspots [3], suggesting the existence of subcellular spatial domains. However, because of the spatial scale at stake, little is currently known about the mechanisms that regulate Ca2+ signaling in fine processes. Here, we investigate the geometry of the endosplamic reticulum (ER), the predominant astrocytic Ca2+ store, using electron microscopy. Contrary to previous reports [4], we detect ER in PAPs, which can be as close as ~60nm to the closest postsynaptic density. We use computational modeling to investigate the impact of the observed cellular and ER geometries on Ca2+ signaling. Simulations using the stochastic voxel-based model from Denizot et al [5], both in simplified and in realistic 3D geometries, reproduce spontaneous astrocytic microdomain Ca2+ transients measured experimentally. In our simulations, the effect of the clustering of IP3R channels observed in 2 spatial dimensions [5] is still valid in a simple cylinder geometry but no longer holds in complex realistic geometries. We propose that those discrepancies might result from the geometry of the ER and that, in 3 spatial dimensions, the effects of molecular distributions (such as e.g IP3R clustering) are particularly enhanced at ER- plasma membrane contact sites. Our results suggest that the predictions from simulations in 1D, 2D or simplified 3D geometries should be cautiously interpreted. Overall, this work provides a better understanding of IP3R-dependent Ca2+ signals in fine astrocytic processes and more generally in subcellular compartments, a prerequisite for understanding the dynamics of Ca2+ hotspots, which are deemed essential for local intercellular communication.
References [1] Bindocci, E., Savtchouk, I., Liaudet, N., et al. “Three-dimensional Ca2+ imaging advances understanding of astrocyte biology,” Science, May 2017, vol. 356, no. 6339, p. eaai8185. [2] Savtchouk, I., Volterra, A. “Gliotransmission: Beyond Black-and-White,” J. Neurosci., Jan. 2018, vol. 38, no. 1, pp. 14–25.
[3] Thillaiappan, N. B., Chavda, A., Tovey, S., et al. “Ca2+ signals initiate at immobile IP3 receptors adjacent to ER-plasma membrane junctions,” Nat. Commun., Dec. 2017 , vol. 8.
[4] Patrushev, I., Gavrilov, N., Turlapov, V., et al. “Subcellular location of astrocytic calcium stores favors extrasynaptic neuron-astrocyte communication,” Cell Calcium, Nov. 2013, vol. 54, no. 5, pp. 343–349.
[5] Denizot, A., Arizono, M., Nägerl, U. V., et al. “Simulation of calcium signaling in fine astrocytic processes: Effect of spatial properties on spontaneous activity,” PLOS Comput. Biol., Aug. 2019, vol. 15, no. 8, p. e1006795.
