Spinal cord injury induces endoplasmic reticulum stress with different cell-type dependent response Clara Penas,* Mo ´ nica-Sofı ´a Guzma ´n,* Enrique Verdu ´ ,* Joaquim Fore ´s, Xavier Navarro* and Caty Casas* *Group of Neuroplasticity and Regeneration, Institute of Neurosciences and Department of Cell Biology, Physiology and Immunology, Universitat Auto `noma de Barcelona, Bellaterra, Barcelona, Spain  Hand and Peripheral Nerve Unit, Hospital Clı ´nic i Provincial, Universitat de Barcelona, Barcelona, Spain Abstract The mechanisms of injury-induced apoptosis of neurons within the spinal cord are poorly understood. In this study, we show that spinal cord injury (SCI) induces endoplasmic reticulum stress revealed by the activation of an unbalanced unfolded protein response (UPR). Using a weight-drop con- tusion model of SCI, the UPR activation was characterized by a quick transient phosphorylation of alpha subunit of euk- aryotic initiation factor 2 soon restored by the up-regulation of its regulator Gadd34; an effective cleavage/activation of the ATF6a transcription factor leading to up-regulation of the canonical UPR target genes Chop,Xbp1 and Grp78; the presence of the processing of Xbp1 mRNA indicative of inositol requiring kinase 1 activation, and a gradual accu- mulation of C/EBP homologous transcription factor protein (CHOP) with concomitant caspase-12 activation. Interestingly, the subcellular distribution of CHOP was found in the nucleus of neurons and oligodendrocytes but in the cytoplasm of astrocytes. Considering the pro-apoptotic action attributed to this transcription factor, this phenomenon might account for the different susceptibility of cell types to dye after SCI. Keywords: apoptosis, astrocytes, endoplasmic reticulum stress, neuronal death, oligodendrocytes, spinal cord injury. J. Neurochem. (2007) 102, 1242–1255. Spinal cord injury (SCI) represents a severe health problem associated with life-long disability. SCI evolves through three phases: the acute, mostly due to mechanical damage, the secondary and the chronic phases (Tator and Fehlings 1991). Downstream events following primary injury involve a complex cascade of molecular events, culmin- ating in a progressive degenerating process termed ‘secon- dary injury’ to the spinal cord (Tator and Fehlings 1991; Lu et al. 2000). Secondary injury processes include distur- bances in ionic homeostasis, local edema, focal hemor- rhage, excitotoxicity, presence of free radicals and free fatty acids and activation of an inflammatory response (Park et al. 2004). Several reports have suggested that apoptosis plays a pivotal role in this secondary damage in animal models and in human tissue (Katoh et al. 1996; Li et al. 1996; Crowe et al. 1997; Liu et al. 1997; Shuman et al. 1997; Emery et al. 1998). However, little evidence has been presented supporting the relation of specific pathways to the induction of neuronal apoptosis after SCI. Identifying specific mechanisms of secondary injury among overlap- ping molecular pathways is further complicated by neur- onal–glial interactions. Furthermore, neurons and oligodendrocytes are specifically and severely affected by SCI. In contrast, astrocytes survive and even proliferate after SCI, becoming reactive (Baldwin et al. 1998; Eng Received October 7, 2006; revised March 13, 2007; accepted March 16, 2007. Address correspondence and reprint requests to Caty Casas, Unitat de Fisiologia Me `dica, Edif. M, Universitat Auto `noma de Barcelona, E-08193 Bellaterra, Spain. E-mail: caty.casas@uab.es Abbreviations used: ATF6a, activating transcription factor 6a; BBB, Basso, Beattie, Bresnahan; CHOP, C/EBP homologous transcription factor; eIF2a, alpha subunit of eukaryotic initiation factor 2; ER, endoplasmic reticulum; ERSE, ER stress elements; Gadd34, growth arrest and DNA damage inducible gene 34; Gapdh, glyceraldehyde-3- phosphate dehydrogenase; Grp78, glucose regulated-protein; IRE1, inositol requiring kinase 1; PERK, RNA-activated protein kinase-like ER resident kinase; SCI, spinal cord injury; Tm, Tunicamycin; UPR, unfolded protein response; XBP1, X-box-binding protein-1. Journal of Neurochemistry , 2007, 102, 1242–1255 doi:10.1111/j.1471-4159.2007.04671.x 1242 Journal Compilation Ó 2007 International Society for Neurochemistry, J. Neurochem. (2007) 102, 1242–1255 Ó 2007 The Authors