nor-1 Regulates Hippocampal Axon Guidance, Pyramidal Cell Survival, and Seizure Susceptibility

nor-1 (NR4A3) is a member of the nuclear receptor family of transcription factors, whose developmental and physiological functions are poorly understood. nor-1 (NR4A3) and two highly homologous transcription factors, nurr1 (NR4A2) and nur77 (NR4A1), constitute the NR4A subfamily of nuclear receptors (2). Unlike most nuclear receptors, the regulator function of NR4A receptors is constitutively active and does not require ligand modulation (17, 40, 43). All three proteins are products of immediate-early genes whose transcription is rapidly induced in response to a variety of extracellular stimuli, including mitogens, apoptotic signals, and neurotransmitters (reviewed in reference 21). The induced nuclear receptors undergo site-specific phosphorylation to modulate their transcriptional activity (8, 14, 27) and bind to specific cis-acting DNA elements containing the sequence motif AAAGGTCA to regulate the expression of defined sets of target genes in a stimulus- and cell-specific manner (17, 26, 43). Previous studies have suggested that members of the NR4A subfamily of nuclear receptors, including nor-1, play key roles in mediating neuronal differentiation during central nervous system (CNS) development and in activity-dependent maintenance of neuronal plasticity in the adult CNS. For example, expression of nurr1 in the ventral midbrain is essential for embryonic specification of dopaminergic neurons (5, 33, 46). All three NR4A receptors are expressed in the limbic system, including hippocampal pyramidal neurons of the adult CNS (32, 47), suggesting a potential involvement in the regulation of long-term changes in gene expression associated with learning and memory. In addition, their expression is rapidly and transiently induced in both pyramidal neurons and dentate granule cells of the hippocampus in experimental animal models of temporal lobe epilepsy (TLE), including chemically and electrically induced limbic seizure activity (7, 28, 42, 44). Pathological seizure activity in both human TLE and experimental rodent models of TLE is associated with region-specific cell death and lasting synaptic reorganization of hippocampal circuitry of the mature CNS (3, 4, 39). These changes require new protein synthesis and are associated with a rapid upregulation of immediate-early gene products, among which are several classes of transcription factors whose activity is thought to mediate the long-term changes in synaptic plasticity associated with pathological seizure activity (22). Consistent with this hypothesis, several members of the AP-1 class of transcription factors, including c-jun and c-fos, are induced by seizures, and inhibition of c-fos (41) or the c-jun-activating kinase jnk3 (45) results in a reduction in seizure activity and associated pathological synaptic plasticity. The rapid seizure-dependent upregulation of NR4A receptors in the hippocampus predicts that this subfamily of nuclear receptor transcription factors may also contribute to the regulation of specific gene programs whose function is to modulate activity-dependent long-term synaptic plasticity. However, elucidation of the specific roles of these receptors in hippocampal plasticity is complicated by their overlapping spatiotemporal expression and activity-dependent induction patterns in the adult hippocampus and by their ability to regulate overlapping genes through interaction with common cis-acting DNA elements. The potential for functional redundancy among NR4A nuclear receptors in mediating epileptogenesis is also underscored by the observation that the development of electrically induced seizure activity (kindling) and its associated synaptic reorganization are unaffected in mice lacking nur77 (48). To further explore the role of NR4A nuclear receptors in the hippocampus, we have analyzed the selective contribution of nor-1 to hippocampal development and seizure-associated synaptic plasticity by examination of the consequences of its elimination in mice. Previously, our laboratory demonstrated that null mutation of the nor-1 gene in mice (nor-1−/−) did not affect postnatal viability but resulted in abnormal vestibular function due to defective outgrowth of the semicircular canals of the inner ear (30). In the present study, we demonstrate that inhibition of nor-1 expression also results in defective axonal growth and region-specific cell death in the hippocampus during the early postnatal stages of development. The developmental defects persist to adulthood and are associated with lasting changes in hippocampal excitability and increased susceptibility of nor-1−/− mice to chemically induced seizures.