Local generation of glia is a major astrocyte source in postnatal cortex

Abstract

Glial cells constitute nearly 50% of the cells in the human brain. Astrocytes, which make up the largest glial population, are crucial to the regulation of synaptic connectivity during postnatal development. Because defects in astrocyte generation are associated with severe neurological disorders such as brain tumours, it is important to understand how astrocytes are produced. Astrocytes reportedly arise from two sources: radial glia in the ventricular zone and progenitors in the subventricular zone, with the contribution from each region shifting with time. During the first three weeks of postnatal development, the glial cell population, which contains predominantly astrocytes, expands 6-8-fold in the rodent brain. Little is known about the mechanisms underlying this expansion. Here we show that a major source of glia in the postnatal cortex in mice is the local proliferation of differentiated astrocytes. Unlike glial progenitors in the subventricular zone, differentiated astrocytes undergo symmetric division, and their progeny integrate functionally into the existing glial network as mature astrocytes that form endfeet with blood vessels, couple electrically to neighbouring astrocytes, and take up glutamate after neuronal activity.

Figure 1. Locally generated glia as a major source of astrocytes

a, Procedure to label SVZ/radial glia-derived astrocytes by electroporation. b, The distribution of astrocytes (arrows) 2 weeks after electroporation. VZ, ventricular zone. c, Percentages of astrocytes at different locations. WM, white matter. d, Proliferating cells (Ki67⁺, red) in a cortical section of P3 mouse. Nuclei were stained with 4′,6-diamidino-2-phenylindole (DAPI, blue). e, Procedure to label locally proliferating cells by retrovirus. f, Cells labelled by retrovirus (green). g, Image of infected astrocytes. Astrocytes (BLBP⁺, red) with GFP (GFP⁺BLBP⁺) or without GFP (GFP⁻BLBP⁺) in the outlined region (dashed line) were included for analysis in h. RV, retrovirus. h, Percentages of astrocytes labelled by retrovirus injected locally, calculated as 100 ×(BLBP⁺GFP⁺ cells/BLBP⁺ cells). Scale bars, 200 μm (b), 50 μm (d), 500 μm (f) and 40 μm (g).

Figure 2. Properties of dividing cells within the cortex

a, Nuclei (arrowheads, Hoechst 33342) of dividing cells at different mitotic stages in acute brain slices. b, A dividing cell (differential interference contrast, arrow) at prometaphase (Hoechst 33342, arrowhead). c–e, Voltage responses from an Astro-like-D cell (c), a dividing NG2 glia (d) and a dividing SVZ cell (e). f–i, Current responses from a non-dividing (ND) astrocyte (f), an Astro-like-D cell (g), a dividing NG2 glia (h) and a dividing SVZ cell (i) evoked by step voltages (inset, g). j, Current–voltage curves in f–i (the circles indicate the positions of measurements). k, Astro-like-D cells (arrows; two at telophase, one at metaphase) stained with anti-GFAP (red). l, m, Morphology of a non-dividing astrocyte (arrow) and an Astro-like-D cell (arrowhead, l) in the cortex, and dividing cells (arrowheads) in the SVZ (m) of a P8 hGFAP-CreER;Ai14 transgenic mouse. n, Summary of the area covered by the processes of non-dividing astrocytes (grey), Astro-like-D cells (red) and SVZ dividing cells (blue) (10-μm z-projection with soma included, one-way analysis of variance followed by a Bonferroni post-hoc test; two asterisks, P < 0.01). o, A non-dividing astrocyte (arrow) was loaded with biocytin. A dividing astrocyte is labelled by biocytin (arrowhead). tg, transgenic. Scale bars, 5 μm (a) and 10 μm (b, k–m, o). Error bars indicate s.e.m.

Figure 3. Time-lapse imaging of local proliferation of astrocytes

a–c, Proliferating astrocytes (arrowheads) in the cortex of hGFAP-GFP transgenic mice, at P3 (a), P6 (b) and P14 (c). d, Summarized data for the percentage of Ki67⁺GFP⁺ cells among GFP⁺ cells with strong GFP signals. e, f, Sequential images of a cortical slice from a P3 hGFAP-GFP transgenic mouse (e, parent cells (arrows); f, daughter cells (arrowheads)). g, Time-lapse images (1 h 52 min) of a dividing GFP⁺ cell in e and f. h, Procedure to image cell division in vivo. i–k, Images from a P4 triply transgenic hGFAP-CreER;Ai14;CAG-Fucci-Green mouse (i, combined images; j, tdTomato; k, mAG signal; arrowheads, dividing astrocytes). l, m, Time-lapse images at 1 h 35 min (l) and 18 h 58 min (m) from a P5 hGFAP-CreER;Ai14 transgenic mouse (arrowheads, dividing astrocytes). Scale bars, 40 μm (a–g) and 100 μm (k, m).

Figure 4. Symmetric division of proliferating astrocytes and the function of their progeny

a, A pair of daughter astrocytes (arrowheads) at late telophase under differential interference contrast. b, Both cells had GFP signal. Nuclei were stained with Hoechst 33342 (HO, inset). c, Voltage responses of two daughter cells evoked by step currents (−1 to 6 nA). d, Two daughter cells in telophase (arrowheads) from a P8 hGFAP-CreER;Ai14 transgenic mouse. e, An astrocyte infected by GFP-expressing retroviruses (green) and expressing tdTomato (red) formed endfeet (arrows) with blood vessels (Laminin⁺, purple) in a P19 hGFAP-CreER;Ai14 transgenic mouse. Tamoxifen was injected at P2, and cells were infected with retrovirus at P5. f, The percentage of progeny cells marked by retroviruses (GFP⁺tdTomato⁺) that had endfeet (GFP⁺tdTomato⁺ with endfeet, blue). g, A retrovirus-infected astrocyte progeny (GFP⁺, green, arrow) in the absence (Ctrl, upper) or presence (lower) of 100 μM carbenoxolene (CBX) was injected with biocytin (red). Without CBX, both GFP⁺ astrocytes (arrowheads) and GFP⁻ astrocytes (asterisks) contained biocytin (red), as a result of gap-junction coupling with the astrocyte progeny injected with biocytin. h, The number of cells coupled. Two asterisks, P < 0.01, (unpaired t-test). i, Current responses of uninfected (GFP⁻) and infected (GFP⁺) astrocyte progeny. j, k, Glutamate transporter current (j) and its summarized data (k) from infected astrocyte progeny before (black) and after (red) application of blocker TBOA (100 μM, 70.4 ±5.3%, n =7). Two asterisks, P < 0.01 (paired t-test). Scale bars, 10 μm (a, b, d, e) and 20 μm (g). Error bars indicate s.e.m.