Fig. 1. EphA4 mutants and the thalamocortical somatosensory map. (A,B) The point-to-point topography of projections between the thalamic primay somatosensory nucleus VB [subdivided into a medial and a lateral part, ventro-postero-medial (VPM) and ventro-postero-lateral (VPL), respectively) and the primary somatosensory area S1, and its relation to ephrin/Eph gradients. Schematic drawings and a low-power view are shown. In control wild-type (WT) mice (B,C), focal injection of DiI in S1 resulted in the retrograde labeling of a single cluster of cells in the thalamic nucleus VPM (asterisk in C). In addition, labeling of the postero-medial nucleus (PoM), a secondary somatosensory nucleus, can be detected in some cases outside the VB, but its diffuse and variable pattern was not analyzed in this study. (D) By contrast, the EphA4^KO/KO mutants show, in addition to the normal cluster, ectopic cells that are located more medially than their normal location (arrows), indicating that thalamocortical projections are not properly organized. Scale bar in C: 400 µm for B-D. (E) Schematic model of the intracellular domains of wild-type and mutant forms of EphA4 encoded by the mutant alleles examined in this study. In KD, EphA4 lysine residue K653 (K) in the kinase catalytic domain was replaced by a methionine (M), rendering the receptor kinase domain completely inactive. In EE mutants, the two juxtamembrane residues (Y) are replaced by two glutamic acid residues (E), rendering the receptor kinase activity constitutively active. In SAM mutants, the entire SAM domain (gray oval) is deleted. In PBM mutants, the entire PBM (black oval) is deleted. In GFP alleles, the entire intracellular domain is replaced by GFP (large gray oval). (F) Expression of wild-type and mutant EphA4 protein in the thalamus of the indicated mutant mice. Protein (50 µg) from thalamic extracts of the indicated mutant mice was resolved by SDS-PAGE and analyzed by western blot using an anti-EphA4 antibody raised against the extracellular domain of the protein (upper panels); the blot was reprobed with an anti-tubulin antibody for loading control (lower panels). EphA4 expression was analyzed in at least two independent animals per genotype. The lanes are representative expression examples of the different genotypes, and all of them belong to the same gel and the same western blot analysis.