Cortical inhibitory circuits are formed by γ-aminobutyric acid (GABA)-secreting interneurons, a cell population that originates far from the cerebral cortex in the embryonic ventral forebrain. Given their distant developmental origins, it is intriguing how the number of cortical interneurons is ultimately determined. One possibility, suggested by the neurotrophic hypothesis1,2,3,4,5, is that cortical interneurons are overproduced, and then after their migration into cortex the excess interneurons are eliminated through a competition for extrinsically derived trophic signals. Here we characterize the developmental cell death of mouse cortical interneurons in vivo, in vitro and after transplantation. We found that 40% of developing cortical interneurons were eliminated through Bax (Bcl-2-associated X)-dependent apoptosis during postnatal life. When cultured in vitro or transplanted into the cortex, interneuron precursors died at a cellular age similar to that at which endogenous interneurons died during normal development. Over transplant sizes that varied 200-fold, a constant fraction of the transplanted population underwent cell death. The death of transplanted neurons was not affected by the cell-autonomous disruption of TrkB (tropomyosin kinase receptor B), the main neurotrophin receptor expressed by neurons of the central nervous system6,7,8. Transplantation expanded the cortical interneuron population by up to 35%, but the frequency of inhibitory synaptic events did not scale with the number of transplanted interneurons. Taken together, our findings indicate that interneuron cell death is determined intrinsically, either cell-autonomously or through a population-autonomous competition for survival signals derived from other interneurons.
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We thank Y. Yanagawa for GAD67–GFP mice, L. Reichardt for TrkB mutant mice and R. Romero for technical contributions. D.G.S. and M.F.P. were supported by training grants from the California Institute for Regenerative Medicine. J.Y.S. was supported by a fellowship from the National Institute of Neurologic Disorders and Stroke (F32NS061497). This work was supported by the California Institute for Regenerative Medicine (A.A.-B., TR2-01749), the John G. Bowes Research Fund (A.A.-B.), the Spanish Ministry of Science and Innovation (J.M.G-V., SAF-2008-01274), and the National Institutes of Health (J.L.R., S.C.B. and A.A.-B., R01 NS071785; A.A.-B., R01 NS048528).
The authors declare no competing financial interests.
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Southwell, D., Paredes, M., Galvao, R. et al. Intrinsically determined cell death of developing cortical interneurons. Nature 491, 109–113 (2012) doi:10.1038/nature11523
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