RTP801 Is Involved in Mutant Huntingtin-Induced Cell Death Núria Martín-Flores 1 & Joan Romaní-Aumedes 1 & Laura Rué 2,3,4 & Mercè Canal 1 & Phil Sanders 2,3,4 & Marco Straccia 2,3,4 & Nicholas D. Allen 5 & Jordi Alberch 2,3,4 & Josep M. Canals 2,3,4 & Esther Pérez-Navarro 2,3,4 & Cristina Malagelada 1 Received: 16 January 2015 /Accepted: 30 March 2015 # Springer Science+Business Media New York 2015 Abstract RTP801 expression is induced by cellular stress and has a pro-apoptotic function in non-proliferating differen- tiated cells such as neurons. In several neurodegenerative dis- orders, including Parkinson’ s disease and Alzheimer’ s dis- ease, elevated levels of RTP801 have been observed, which suggests a role for RTP801 in neuronal death. Neuronal death is also a pathological hallmark in Huntington’ s disease (HD), an inherited neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene. Currently, the exact mechanisms underlying mutant huntingtin (mhtt)-induced toxicity are still unclear. Here, we investigated whether RTP801 is involved in (mhtt)-induced cell death. Ectopic exon-1 mhtt elevated RTP801 mRNA and protein levels in nerve growth factor (NGF)-differentiated PC12 cells and in rat primary cortical neurons. In neuronal PC12 cells, mhtt also contributed to RTP801 protein elevation by reducing its proteasomal degradation rate, in addition to promoting RTP801 gene expression. Interestingly, silencing RTP801 ex- pression with short hairpin RNAs (shRNAs) blocked mhtt- induced cell death in NGF-differentiated PC12 cells. However, RTP801 protein levels were not altered in the stria- tum of Hdh Q7/Q111 and R6/1 mice, two HD models that dis- play motor deficits but not neuronal death. Importantly, RTP801 protein levels were elevated in both neural telence- phalic progenitors differentiated from HD patient-derived in- duced pluripotent stem cells and in the putamen and cerebel- lum of human HD postmortem brains. Taken together, our results suggest that RTP801 is a novel downstream effector of mhtt-induced toxicity and that it may be relevant to the human disease. Keywords RTP801 . Exon-1 mutant huntingtin . HdH Q7/Q111 mice . Putamen . R6/1 mice . Striatum . PC12 cells . Neuron death Introduction RTP801/REDD1 protein, encoded by the DDIT4 gene, was first identified on the basis of its induction by hypoxia [1] and DNA damage [2]. Other stressors induced its expression such as dexamethasone, thapsigargin, tunicamycin, heat shock in murine T cell lymphoma cells [3], or cigarette smoke in lung cells [4]. In the central nervous system, RTP801 expression is in- creased in response to ischemia, β-amyloid peptide [5, 6], 6- hydroxydopamine (6-OHDA) [7, 8], and 1-methy-4-phenyl-1, 2,3,6-tetrahydropyridine [8]. Importantly, RTP801 not only accumulates in cellular and animal toxic models but also in * Esther Pérez-Navarro estherperez@ub.edu * Cristina Malagelada crismalagelada@gmail.com 1 Department of Pathological Anatomy, Pharmacology and Microbiology, Faculty of Medicine, University of Barcelona, Casanova 143, 08036 Barcelona, Catalonia, Spain 2 Department of Cell Biology, Immunology and Neurosciences, Faculty of Medicine, University of Barcelona, Casanova 143, 08036 Barcelona, Catalonia, Spain 3 Institut d’Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), 08036 Barcelona, Catalonia, Spain 4 Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas (CIBERNED), Madrid, Spain 5 Divisions of Pathophysiology & Repair and Neuroscience, School of Biosciences, Cardiff University, Cardiff CF10 3AX, UK Mol Neurobiol DOI 10.1007/s12035-015-9166-6