G subunits of heterotrimeric G proteins are required for proper mitotic-spindle orientation and cell fate determination in the developing neocortex.
Orientation of cell-cleavage planes relative to the ventricular zone in the mammalian neocortex is important for the differentiation of progenitor cells into projection neurons, but little is known about how it is regulated. Reporting in Cell, Sanada and Tsai show that G protein subunits (G) are important for regulating mitotic spindle orientation and cell fate determination during neurogenesis in the cerebral cortex.
Brain sections stained with antibodies against the DNA synthesis marker BrdU (red), the cell cycle marker Ki67 (blue) and green fluorescent protein (GFP; green). GFP-positive cells labelled with both BrdU and Ki67 (white; arrowheads) are active in the cell cycle, whereas GFP-positive cells labelled with BrdU but not Ki67 (yellow; arrows) have exited the cell cycle. Image courtesy of L.-H. Tsai, Howard Hughes Medical Institute, Harvard Medical School, Massachusetts, USA.
Progenitor cells at the ventricular zone are highly polarized along the basal–apical axis, and it is thought that determinants of neuronal fate might be asymmetrically located at the basal side. When progenitors divide with the cleavage plane perpendicular to the ventricular surface (vertical cleavage plane), the two daughter cells have the same cellular determinants. Before neurogenesis, this symmetrical division produces two progenitors and expands the pool of cells that will ultimately give rise to neurons. However, when progenitors divide with the cleavage plane parallel to the ventricular surface (horizontal cleavage plane), the determinants are predominantly segregated into the basal daughter cells, and thereby induce asymmetric cell fates.
In this study, the researchers first established that about 50% of the dividing cells in the neocortex of mouse embryos had the vertical cleavage plane. This percentage increased to 72% when an inhibitor of G, the carboxy-terminal region of -adrenergic receptor kinase (ARK-ct), was overexpressed. Overexpression of Gi, which is known to sequester free G and thereby inhibit its signalling, had a similar effect.
Interestingly, overexpression of either ARK-ct or Gi resulted in a significant increase in the number of differentiated cortical neurons, which indicates that altered orientation in cleavage planes led to changes in cell fate. Further analysis showed that there was an increase in the percentage of progenitors that exited the cell cycle, which was correlated with a decrease in the number of mitotic cells.
Which G-protein activators are responsible for mitotic spindle orientation? Classically, ligands that bind to G-protein-coupled receptors (GPCRs) initiate the cascade of downstream signalling. However, Sanada and Tsai found that a GPCR-independent activator of G proteins, AGS3, was important for establishing the axis of division in cortical progenitors. AGS3 is expressed by cortical progenitors in mouse embryos, and silencing its expression causes abnormalities in the mitotic spindle orientation that are similar to those caused by disruption of G signalling.
This elegant study provides the first direct evidence for the molecular mechanism that regulates mitotic spindle orientation in cortical progenitor cells. It will lead to new ways of deciphering the complex events that trigger changes in cell-cleavage plane orientation and asymmetric cell fate choices during neurogenesis.
Jane Qiu
References
Sanada, K. & Tsai, L.-H. G protein subunits and AGS3 control spindle orientation and asymmetric cell fate of cerebral cortical progenitors. Cell122, 119–131 (2005) | Article | PubMed |
Götz, M. & Huttner, W. B. The cell biology of neurogenesis. Nature Rev. Mol. Cell Biol. (in the press)
subunits of heterotrimeric G proteins are required for proper mitotic-spindle orientation and cell fate determination in the developing neocortex.
i, which is known to sequester free G