Knockdown of Nurr1 in the rat hippocampus: implications to spatial discrimination learning and memory

Expression pattern of NuIP gene in adult mouse brain

We previously reported the identification of the Nurr1 interacting protein (NuIP) that was demonstrated to modulate the transcriptional activity of Nurr1, the orphan nuclear receptor required for midbrain dopaminergic neuron differentiation. NuIP was also cloned by others and referred to as a small G protein signaling modulators. The open reading frame of NuIP predicts a protein with an N-terminal RUN domain (RPIP8, UNC-14, and NESCA) and a C-terminal TBC domain (Tre-2, Bub2, and Cdc16) both of which are found in proteins of the GTPase activating protein (GAP) family, involved in the GTPase signaling pathway. To characterize the NuIP gene product, we developed a polyclonal antibody. Since NuIP gene is expressed most abundantly in adult and the level of expression during development is below the detection limit of immunohistochemistry, we now report the expression pattern of NuIP in adult mouse brain compared with the expression pattern of Nurr1 protein. Many regions co-expressed Nurr1 and NuIP including cortex, hippocampus, substantia nigra, and the cerebellum. However, there are also regions that exclusively express NuIP such as striatum, septum, globus pallidus, and the reticular thalamic nucleus. We also find that NuIP protein expresses mainly in NeuN-positive (neuronal nuclei) neurons but can be detected in GFAP-positive (glial fibrillary acidic protein) glial cells in hippocampus. Interestingly, NuIP is expressed in high levels in midbrain dopaminergic neurons including ventral tegmental area (VTA) and substantia nigra (SN) dopaminergic neurons but is not expressed or expressed in low levels in other dopaminergic neurons such as olfactory bulb and hypothalamus. Overall, the expression pattern of NuIP in adult mouse brain suggests that it may be involved in motor activity control in basal ganglia as well as higher central nervous system (CNS) functions such as cognition and memory in cortex and hippocampus.

Identification of flap structure-specific endonuclease 1 as a factor involved in long-term memory formation of aversive learning

We previously proposed that DNA recombination/repair processes play a role in memory formation. Here, we examined the possible role of the fen-1 gene, encoding a flap structure-specific endonuclease, in memory consolidation of conditioned taste aversion (CTA). Quantitative real-time PCR showed that amygdalar fen-1 mRNA induction was associated to the central processing of the illness experience related to CTA and to CTA itself, but not to the central processing resulting from the presentation of a novel flavor. CTA also increased expression of the Fen-1 protein in the amygdala, but not the insular cortex. In addition, double immunofluorescence analyses showed that amygdalar Fen-1 expression is mostly localized within neurons. Importantly, functional studies demonstrated that amygdalar antisense knockdown of fen-1 expression impaired consolidation, but not short-term memory, of CTA. Overall, these studies define the fen-1 endonuclease as a new DNA recombination/repair factor involved in the formation of long-term memories.

mRNA expression of the Nurr1 and NGFI-B nuclear receptor families following acute and chronic administration of methamphetamine

Nur-related 1 (Nurr1) and nerve growth factor inducible-B (NGFI-B) constitute closely related subgroups of the nuclear receptor superfamily. One to three hours after 4 mg/kg acute methamphetamine (METH) administration, the levels of Nurr1 mRNA were significantly higher in the prelimbic (PrL), primary motor (M1) and primary somatosensory (S1) cortices and ventral tegmental area (VTA), as compared with the basal level. Pretreatment with 0.5 mg/kg of SCH23390 prevented the acute METH-induced increase in Nurr1 mRNA levels in these brain regions. One to three hours after 4-mg/kg acute METH administration, the levels of NGFI-B mRNA increased significantly in the PrL, M1, S1, striatum, and nucleus accumbens core (AcbC). Pretreatment with either 0.5 mg/kg of MK-801 or 0.5 mg/kg of SCH23390 prevented the acute METH-induced increase in NGFI-B mRNA levels in these brain regions. The levels of mRNAs were determined 3 h after a challenge injection of either saline or 4 mg/kg METH at the three-week withdrawal point in rats which had previously been exposed to either saline or METH (4 mg/kg/day) for 2 weeks. After the saline challenge, the group chronically exposed to METH displayed significantly higher levels of Nurr1 mRNA in the PrL, S1 and VTA, and of NGFI-B mRNA in the PrL, M1, S1, striatum and AcbC than did the group chronically treated with saline. The groups chronically exposed to METH failed to increase Nurr1 mRNA in the VTA, and NGFI-B mRNA in the AcbC, when challenged with 4 mg/kg METH. These results suggest that Nurr1 and NGFI-B mRNA play differential roles upon exposure to METH.

Neuroinflammation and cognitive function in aged mice following minor surgery

Following surgery, elderly patients often suffer from postoperative cognitive dysfunction (POCD) which can persist long after physical recovery. It is known that surgery-induced tissue damage activates the peripheral innate immune system resulting in the release of inflammatory mediators. Compared to adults, aged animals demonstrate increased neuroinflammation and microglial priming that leads to an exaggerated proinflammatory cytokine response following activation of the peripheral immune system. Therefore, we sought to determine if the immune response to surgical trauma results in increased neuroinflammation and cognitive impairment in aged mice. Adult and aged mice underwent minor abdominal surgery and 24h later hippocampal cytokines were measured and working memory was assessed in a reversal learning version of the Morris water maze. While adult mice showed no signs of neuroinflammation following surgery, aged mice had significantly increased levels of IL-1beta mRNA in the hippocampus. Minor surgery did not result in severe cognitive impairment although aged mice that underwent surgery did tend to perseverate in the old target during reversal testing suggesting reduced cognitive flexibility. Overall these results suggest that minor surgery leads to an exaggerated neuroinflammatory response in aged mice but does not result in significantly impaired performance in the Morris water maze.

Serum- and glucocorticoid-inducible kinase 1 enhances zif268 expression through the mediation of SRF and CREB1 associated with spatial memory formation

Serum- and glucocorticoid-inducible kinase 1 (SGK1) has been shown to play an important role in spatial memory formation, but the molecular mechanism underlying this effect of SGK1 was not known. zif268 is an immediate early gene that is induced by water maze learning. To investigate the role of SGK1 in the regulation of zif268 expression, the dominant negative mutant of SGK1, SGK1 S422A, was infused to the hippocampal CA1 area of rats, and was found to decrease significantly the mRNA level of zif268 in both naïve animals and trained animals. SGK1 was also found to phosphorylate serum response factor (SRF) at Ser73, Ser75, and Ser99, and phosphorylate CREB1 at Ser133. Inhibition of SGK1 phosphorylation sites on SRF and CREB1 with alanine substitution significantly diminished SGK1-enhanced zif268 expression in the promoter-luciferase assay. SGK1 also phosphorylates Elk-1 and SGK1 phosphorylation of Elk-1 decreased the transcriptional activity of Elk-1. But SGK1 phosphorylation of Elk-1 did not affect SGK1-enhanced zif268 expression. Moreover, the phosphorylation of SGK1 was increased in rat CA1 area after water maze learning, accompanied by increased phosphorylation of SRF at Ser99 and increased phosphorylation of CREB1 at Ser133. All these effects were antagonized by SGK1 S422A transfection. These results together suggest that SGK1 enhances zif268 expression through the mediation of SRF and CREB1, and these signaling pathways are associated with spatial memory formation in rats.

The glucocorticoid receptor is a co-regulator of the orphan nuclear receptor Nurr1

Nurr1 (NR4A2) is an atypical nuclear receptor (NR) because of its inability to bind a ligand and to activate transcription following canonical NR rules. An affinity chromatography-based screen identified the glucocorticoid receptor (GR) as an interactant of Nurr1. The co-localization of these two NRs in the hippocampus and the substantia nigra, as well as their involvement in similar neurological processes led us to investigate the functional consequences of such a physical interaction. GR interfered with Nurr1 transcriptional activity, and Nurr1 association to GR confers glucocorticoid regulation to this orphan receptor. The N-terminal domain of Nurr1 interacts directly with GR, whereas several domains of GR can associate to Nurr1. The GR-mediated increase in Nurr1 transcriptional activity requires the N-terminal domain of GR, but not a functional DNA binding domain. Finally, SMRT and SRC2, two co-regulators of GR, modulated the transcriptional activity of the Nurr1-GR complex, but not that of Nurr1 alone. Our results therefore establish GR as a transcriptional regulator of Nurr1, and open new opportunities in the pharmacological regulation of Nurr1 by glucocorticoids in the CNS.

Histone deacetylase inhibitors enhance memory and synaptic plasticity via CREB:CBP-dependent transcriptional activation

Histone deacetylase (HDAC) inhibitors increase histone acetylation and enhance both memory and synaptic plasticity. The current model for the action of HDAC inhibitors assumes that they alter gene expression globally and thus affect memory processes in a nonspecific manner. Here, we show that the enhancement of hippocampus-dependent memory and hippocampal synaptic plasticity by HDAC inhibitors is mediated by the transcription factor cAMP response element-binding protein (CREB) and the recruitment of the transcriptional coactivator and histone acetyltransferase CREB-binding protein (CBP) via the CREB-binding domain of CBP. Furthermore, we show that the HDAC inhibitor trichostatin A does not globally alter gene expression but instead increases the expression of specific genes during memory consolidation. Our results suggest that HDAC inhibitors enhance memory processes by the activation of key genes regulated by the CREB:CBP transcriptional complex.