Transient Neuronal Populations Are Required to Guide Callosal Axons: A Role for Semaphorin 3C (original) (raw)
Figure 3
CC neuronal network integrity is important for pathfinding by callosal axons.
(A–D) Single immunohistochemistry for SNAP25 (A and B) and double immunohistochemistry for Npn-1 and CR (Ci–Cii and Di–Dii) in coronal CC sections from E16.5 WT (A, Ci, and Cii) and Mash1−/− (B, Di, and Dii) mice. (Cii and Dii) are higher power views of the medial CC seen in (Ci and Di). (A, Ci, and Cii) At E16.5, both callosal and hippocampal commissure (HIC) fibers labeled with SNAP25 or Npn-1 begin to cross the midline and grow towards the contralateral cortex. (B, Di, and Dii) By contrast, in Mash1−/− brains, the majority of callosal and hippocampal fibers do not cross the midline and form large ectopic bundles of axons on either side of it, reminiscent of Probst bundles (PB; open arrowheads). (Di and Dii) The loss of Mash1-positive GABAergic interneurons of the CC causes the disorganization of glutamatergic CR+ neurons at the midline (compare [Dii] with [Cii], arrowheads). (Ei) Experimental paradigm used to confirm the growth of E16.5 GFP+ WT callosal axons in CC transplants and slices from WT mice. (Eii–Eiii) GFP immunocytochemistry showing that WT GFP+ callosal axons grow normally and cross the midline when they are confronted with a WT environment. (Fi) Experimental paradigm used to confirm the growth defects of E16.5 GFP+ Mash1 mutant callosal axons in CC transplants and slices from Mash1−/− mice. (Fii–Fiii) GFP immunocytochemistry showing that GFP+ callosal axons of Mash1−/− cortical explants do not cross the midline, but rather form Probst bundles (PB, open arrowhead). (Gi) Experimental paradigm used to test whether the CC neuronal network integrity is necessary and sufficient to direct the growth of callosal axons. To this end, WT CC is transplanted in a Mash1−/− slice. (Gii–Giii) GFP immunocytochemistry showing the complete restoral of Mash1−/− callosal axons pathfinding. Dashed lines outline the CC transplant localizations. Brain slices in (Eii–Eiii, Fii–Fiii, and Gii–Giii) were counterstained with Hoechst. Bar indicates 435 µm in (Eii, Fii, and Gii), 220 µm in (A, B, Ci, Di, Eiii, Fiii, and Giii), and 110 µm in (Cii and Dii). CFr, frontal cortex.