Organizing Principles of Astrocytic Nanoarchitecture in the Mouse Cerebral Cortex

Abstract

SUMMARYAstrocytes have complex roles in central nervous system (CNS) health and disease. Underlying these roles is an elaborate architecture based on frequent, extremely fine, but seemingly haphazard branches, as well as prominent features including tripartite synaptic complexes and perivascular endfeet. While broad categories of structures in astrocytes are known, the fundamental building blocks that compose them and their organizing principles have yet to be adequately defined. This is largely due to the absence of high-resolution datasets that can reveal nanoscopic features of astrocytes (i.e. 10-20nm diameter in x, y, and z) and a lack of computational approaches that can effectively interrogate astrocyte shape, organization, and nanoarchitecture. Here, we produced and analyzed multiple, high-resolution datasets of layer 2/3 mouse somatosensory cortex using focused ion beam scanning electron microscopy (8nm intervals) and computer vision approaches to provide a principled, quantitative analysis of astrocytic nanoarchitecture. A decomposition of astrocytes into fundamental ‘parts’ led to the discovery of unique structural components, recurring structural motifs, and assembly of parts into an organized hierarchy. New relationships were also discerned between astrocytic processes and other CNS microanatomy including mitochondria, tripartite synapses, and cerebrovasculature. By deploying computational resources to quantitatively understand the organizing principles and nanoarchitecture of astrocytes, this study reveals the specialized anatomical adaptations of these complex cells within the CNS.One Sentence SummaryUsing high-resolution serial electron microscopy datasets and computer vision, this study provides a systematic analysis of astrocytic nanoarchitecture from multiple samples of layer 2/3 of adult mouse neocortex, and presents quantitative evidence that astrocytes organize their morphology into purposeful, classifiable assemblies with unique structural and subcellular organelle adaptations related to their physiological functions.

Astrocyte heterogeneity and interactions with local neural circuits

“…Furthermore, synapses around astrocytes tend to organize into clusters of between 4 and 55 structures, with mitochondria positioned closer to larger synaptic clusters, presumably to match the high energetic demands of synaptic transmission (Figure 4A''). Interestingly, in this study, astrocyte processes were rarely seen looping back on themselves [33]. Hence, astrocytes were proposed to possess a structure largely reminiscent of a 'thorny bush' , rather than a 'sponge' .…”

“…Although the degree of this interaction between astrocytes and neurons varies between brain regions (77% of synaptic interfaces in striatum, 86% in hippocampus, 90% in cortex [28,32]), this structure is obviously remarkably common. An elegant electron microscopy study has recently reconstructed astrocytes in mouse somatosensory cortex with unprecedented resolution, revealing an elaborate heterogeneity in process nanostructure [33]. In this study, processes appear as branches of undulating thickness (comprising both expanded and constricted segments), often ending with a constricted tip that is physically associated with a synapse.…”

“…Both these studies reported 'frequent' loop type structures -although, crucially, neither study quantified the frequency at which these structures appear. At present, the most parsimonious explanation for the discrepancy between these studies is the relatively small number of somatosensory astrocytes reconstructed by Salmon and colleagues [33], combined with potential morphological Heterogeneity in adult astrocytes exists at many levels, including the morphological, transcriptomic/proteomic, physiological and functional. A significant proportion of heterogeneity may exist in the peripheral processes -as illustrated using specific examples discussed in the main text.…”

Control of Ca2+ signals by astrocyte nanoscale morphology at tripartite synapses

“…The ultrastructure of the spongiform domain of astrocytes is very complex, characterized by abundant branching points, conferring a reticular morphology (Arizono et al 2020). Those branching points are reportedly sometimes arranged into ring‐like structures, although their occurrence and shape are still debated and could differ depending on the brain region under study (Arizono et al 2020; Panatier et al 2014; Kiyoshi et al 2020; Salmon et al 2021; Arizono and Nägerl 2021). The effect of this reticular ultrastructure on the propagation of signals remains to be uncovered.…”

Control of Ca2+signals by astrocyte nanoscale morphology at tripartite synapses

“…The ultrastructure of the spongiform domain of astrocytes is very complex, characterized by abundant branching points, conferring a reticular morphology Arizono et al (2020). Those branching points are reportedly sometimes arranged into ring-like structures, although their occurrence and shape are still debated and could differ depending on the brain region under study Arizono et al (2020); Panatier et al (2014); Kiyoshi et al (2020); Salmon et al (2021); Arizono & Nägerl (2021). The effect of this reticular ultrastructure on the propagation of Ca 2+ signals remains to be uncovered.…”