Cell autonomous regulation of MN survival by Met can be achieved through distinct signalling pathways. (A) In vitro assessment of survival of MNs derived from WT, met2P/2P, met2S/2S, and metd/d E12.5 embryos in the presence of GDNF (100 pg/ml). Survival values are expressed as percentage of the survival in basal medium (defined as 100%). The ability of MNs to respond to GDNF is not altered in metd/d, met2P/2P, and met2S/2S mutants. Therefore, in subsequent panels, values are normalised with the survival in basal medium (defined as 0%) and expressed as percentage of their response to GDNF (defined as 100%). (B,C) Survival of brachial + lumbar (B) or thoracic (C) MNs derived from WT, met2P/2P, met2S/2S, and metd/d E12.5 embryos in the presence of HGF (2 ng/ml). Whereas metd/d MNs do not show any survival response to HGF, met2P/2P and met2S/2S MNs respond as efficiently as WT MNs. (D) Signalling requirements downstream of MetWT, Met2P, and Met2S for MN survival by HGF were explored using inhibitors of PI3K (LY294002: 1 μM), Mek (PD98059: 1 μM), and Src (PP2: 0.2 μM), respectively). Inhibitors were used at concentrations not toxic on the basal survival. All three inhibitors efficiently blocked the survival response of WT MNs to HGF. As expected, the survival response in met2P/2P MNs was selectively blocked by the PI3K inhibitor. In contrast, the survival response in met2S/2S MNs was abolished by inhibiting Src or Mek. For each genotype, two to four independent experiments were done in triplicate comparing MNs from mutant embryos and from their WT littermates. (WT: n=9; , met2P/2P: n=3; met2S/2S: n=4; metd/d: n=2). Error bars indicate standard error of the mean. (E) Schematic representation of the signalling components involved in blocking apoptosis downstream of the wild-type MetWT or the specificity switch mutants Met2P and Met2S receptors. The inhibitors used are indicated.