Identification of a Mammalian Homologue of the Fungal Tom70 Mitochondrial Precursor Protein Import Receptor as a Thyroid Hormone-Regulated Gene in Specific Brain Regions Manuel Alvarez-Dolado, *²Marı ´a Gonza ´lez-Moreno, *Alfonso Valencia, ‡Martin Zenke, Juan Bernal, and Alberto Mun ˜oz Instituto de Investigaciones Biome ´dicas “Alberto Sols,” Consejo Superior de Investigaciones Cientı ´ficas (CSIC)–Universidad Auto ´noma de Madrid (UAM), *Centro Nacional de Biotecnologı ´a, Consejo Superior de Investigaciones Cientı ´ficas, Cantoblanco, and ² Centro de Astrobiologı ´a, INTA, Madrid, Spain; and ‡Max-Delbru ¨ck Center for Molecular Medicine, Berlin, Germany Abstract: Thyroid hormone is an important regulator of mammalian brain maturation. By differential display PCR, we isolated a cDNA clone (S2) that is specifically up- regulated in the striatum of neonatal hypothyroid rats. S2 was identified as KIAA0719, the first human gene dis- tantly homologous to the fungal Tom70, which encodes a member of the translocase mitochondrial outer mem- brane complex involved in the import of preproteins into the mitochondria. By northern and in situ hybridization studies, KIAA0719 was found to be up-regulated in the striatum, nucleus accumbens, and discrete cortical layers of 15-day-old hypothyroid rats. In contrast, lower expres- sion was found in the olfactory tubercle, whereas no differences were detected in other brain regions. Signifi- cantly, treatment of hypothyroid animals with single in- jections of thyroxine restored the normal levels of KIAA0719 expression. Moreover, treatment of control an- imals with thyroxine led to a reduced expression, dem- onstrating a negative hormonal regulation in vivo. Thus, KIAA0719 gene expression is regulated by thyroid hor- mone in the neonatal rat brain in a region-specific fash- ion. Given the role of the homologous Tom70 gene, the alteration of KIAA0719 expression may contribute to the changes in mitochondrial morphology and physiology caused by hypothyroidism in the developing rat brain. Key Words: Thyroid hormone —KIAA0719 gene —Mito- chondrial protein import receptor—Tom70 —Striatum— Differential display. J. Neurochem. 73, 2240 –2249 (1999). Thyroid hormone [3,3',5-triiodothyronine (T3); thy- roxine (T4)] is a main regulator of growth, development, and homeostasis in higher organisms. Lack of adequate levels of thyroid hormones during the fetal and neonatal periods leads to mental deficiency in humans and multi- ple brain abnormalities in experimental animals, such as the rat (Legrand, 1984, 1986; Porterfield and Hendrich, 1993; Bernal and Nunez, 1995). These abnormalities include biochemical, metabolic, cellular, and behavioral alterations throughout the brain, including a reduction in the mean size of neuronal cell bodies, a decreased den- sity and abnormal distribution of dendritic spines, changes in the number of particular cellular populations and in synaptogenesis, reduced arborization and delocal- ization of cerebellar Purkinje cells, retarded and dimin- ished myelination, and migration defects (Schwartz and Oppenheimer, 1997; Bernal and Guadan ˜o-Ferraz, 1998). Thyroid hormone acts by regulating target gene ex- pression through binding to its high-affinity receptor [thyroid receptor (TR)], which belongs to the superfam- ily of nuclear hormone receptors that function as ligand- regulated transcription factors (Sap et al., 1986; Wein- berger et al., 1986). The different TR isoforms are dis- tinctly expressed in a time- and region-specific fashion in the developing brain, presumably regulating the tran- scription of a particular set of genes (Mellstro ¨m et al., 1991; Bradley et al., 1992). In the last few years, a number of genes coding for cytoskeletal proteins, tran- scription factors, enzymes, cell-adhesion molecules, neu- rotrophins and their receptors, or other proteins have been found to be under thyroid control in the rodent brain during the neonatal period (for review, see Bernal and Guadan ˜o-Ferraz, 1998). It is, however, clear that addi- tional thyroid hormone target genes must exist, and that their identification will increase our understanding of the molecular events underlying cerebral function and brain development. To identify new genes regulated by thyroid hormone in the brain, we have used the differential display PCR Received May 4, 1999; revised manuscript received August 3, 1999; accepted August 3, 1999. Address correspondence and reprint requests to Dr. A. Mun ˜oz at Instituto de Investigaciones Biome ´dicas “Alberto Sols,” Arturo Dup- erier 4, E-28029 Madrid, Spain. Abbreviations used: Cy, cyclophilin; MAG, myelin-associated gly- coprotein; P, postnatal day; PBS, phosphate-buffered saline; SSC, standard saline citrate; T3, 3,3',5-triiodothyronine; T4, thyroxine; Tom, translocase of the outer mitochondrial membrane; TPR, tetratricopep- tide repeat; TR, thyroid receptor. 2240 Journal of Neurochemistry Lippincott Williams & Wilkins, Inc., Philadelphia © 1999 International Society for Neurochemistry