Mutual inhibition among postmitotic neurons regulates robustness of brain wiring. Marion G Langen^1,2,3, Marta Koch^1,2, Jiekun Yan^1,2, Natalie de Geest^1,2, Marie-Luise Erfurt^4,5, Barret D. Pfeiffer⁶, Dietmar Schmucker^4,5, Yves Moreau⁷, Bassem A. Hassan^1,2,3,4. 1) VIB Center for Biology of Disease, VIB, 3000 Leuven, Belgium; 2) Center for Human Genetics, University of Leuven School of Medicine, 3000 Leuven, Belgium; 3) Doctoral Program in Molecular and Cognitive Neuroscience, Doctoral School of Biomedical Sciences, University of Leuven, 3000 Leuven, Belgium; 4) Vesalius Research Center, VIB, 3000, Leuven, Belgium; 5) Department of Oncology, University of Leuven School of Medicine, 3000 Leuven, Belgium; 6) Janelia Farm Research Campus, Howard Hughes Medical Institute, Ashburn, VA, USA; 7) Bioinformatics Group, Department of Electrical Engineering, University of Leuven, 3000 Leuven, Belgium.

   Brain connectivity maps display a delicate balance between individual variation and stereotypy, suggesting the existence of dedicated mechanisms that simultaneously permit and limit individual variation. We show that mutual inhibition among groups of neighboring postmitotic neurons during development regulates the robustness of axon target choice in a non-deterministic neuronal circuit. Specifically, neighboring postmitotic neurons communicate through Notch signaling during axonal targeting, to ensure balanced alternative axon target choices without a corresponding change in cell fate. Loss of Notch in postmitotic neurons modulates an axons target choice. However, because neighboring axons respond by choosing the complementary target, the stereotyped connectivity pattern is preserved. In contrast, loss of Notch in clones of neighboring postmitotic neurons results in erroneous co-innervation by multiple axons. Our observations establish mutual inhibition of axonal target choice as a robustness mechanism for brain wiring and unveil a novel cell fate independent function for canonical Notch signaling.