A major glial source in the postnatal brain

Glial cells make up approximately 50 percent of the cells in the human brain. Most glial cells are produced after birth: in rodents, their number increases by 6-8-fold within the first 3 postnatal weeks. However, the source that contributes to this glial expansion is unknown. Our study has revealed that astrocytes, the largest glial cell population, are generated post-natally mainly from proliferating glial cells within the cortex rather than progenitors in the subventricular zone (SVZ) or radial glial cells in the ventricular zone (VZ).

HFSP Long-Term Fellow Woo-Ping Ge and colleagues
authored on Fri, 01 June 2012

Radial glial cells or SVZ progenitors give birth to the first wave of astrocytes at late embryonic stages. It was thought that most of the glial cells in the postnatal cortex were also produced in the SVZ/VZ, and then migrated into different cortical layers. Although it has been reported for four decades that local gliogenesis occurs in the cortex, the extent of its contribution to astrocyte generation is unknown.

 

 

Figure: Proliferating astrocytes (yellow, GFP+Ki67+, arrowheads) with green fluorescent protein (GFP+, green) from a transgenic hGFAP-GFP mouse at postnatal day 3.  Proliferating cells are labelled by antibody against Ki67 (Ki67+, red) in a fixed cortical section.

A large number of cells in the SVZ/VZ were labeled by GFP after we electroporated GFP-containing plasmids into the SVZ/VZ regions in the neonatal mice. Two to three weeks later, very few astrocytes generated from these two regions were able to migrate into the surface layers (I-IV) of the cortex; only about 20% of GFP labeled cells migrated into the deep layers (V-VI).  Given that the total astrocyte number increases significantly at this stage, there should be other glial sources in the brain. Taking advantage of retroviral specificity for labeling dividing cells, we injected viruses into the cortex and traced dividing glial cells as well as their progeny for 7-10 days. Surprisingly, nearly 50% of astrocytes in infected regions were labeled by GFP, demonstrating that local production of glia is a major astrocyte source. Consistent with viral labeling results, our time-lapse imaging in ex vivo brain slices and in vivo live mice has provided strong evidence to support that a large number of astrocytes initiated cell division in situ within the 1st postnatal week.

We then used Hoechst 33342 dye to label the live cells that were undergoing mitosis in order to conduct whole-cell patch-clamp recording of those proliferating cells.  Other than proliferating NG2 glial cells that did not differentiate into astrocytes post-natally, the remaining proliferating cells had biophysical properties comparable to those of differentiated mature astrocytes. Different from glial progenitors in the SVZ, locally dividing astrocytes within the cortex underwent symmetric cell division and generated only astrocytes. These locally generated astrocytes integrated into existing glial networks and functioned as mature astrocytes after exiting cell cycle. Very few of them entered programmed cell death.

In summary, our study extends our understanding of astrocyte genesis in the mammalian brain: the first wave of astrocytes is from SVZ progenitors or radial glial cells; after birth, astrocytes in the cortex are mainly produced via local proliferation of astrocytes.  

Reference

Local generation of glia is a major astrocyte source in postnatal cortex. Ge WP, Miyawaki A, Gage FH, Jan YN, Jan LY. Nature. 2012 Mar 28;484(7394):376-80. doi: 10.1038/nature10959.

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