Necdin Protects Motoneurons from Death Figure 5. TNFR1 and Necdin are coexpressed in lumbar motoneurons at E12.5. (A–C) Immunohistochemistry analysis revealing the expression of the death receptors Unc5 (A) or TNFR1 (B and C), on different transversal sections corresponding to the brachial, thoracic and lumbar levels of E12.5 spinal cord. (A) Unc5 (green) is expressed in the brachial ventral horn of the spinal cord. Its expression is fainter in the thoracic ventral horn and is not detected in the lumbar ventral horn. (B) Necdin (red) and TNFR1 (green) are coexpressed in motoneurons of the lumbar region. (C) Higher magnification of (B) showing the colocalization of Necdin with TNFR1. Scale bar: A and B, 100 mm; C, 250 mm. doi:10.1371/journal.pone.0023764.g005 programmed cell death (Figure 8A). Accumulating evidence suggests that motoneurons compete for access to limited quantities of neurotrophic support and that neurotrophic signaling in motoneurons prevents cell death cascades, including activation of caspases. Extrinsic death pathways, that mediate the activation of death signaling cascade, can be triggered by cell surface receptors such as Fas, TNFR, and p75NTR following activation by their respective ligands. TNF a has been shown to play a role in developmental cell death [39], including brachial motoneuron cell death [34]. However during developmental PCD, the pattern of neuronal death in mice lacking TNFR1 is unchanged, suggesting that compensatory mechanisms might take place to complete the developmental program of motoneuron death [40]. Here we show that in the absence of Necdin there is an increase of motoneuronal PLoS ONE | www.plosone.org 7 September 2011 | Volume 6 | Issue 9 | e23764