Enhanced expression of activity-regulated cytoskeleton-associated protein in the medial prefrontal cortex is involved in working memory performance

Working memory (WM) is a cognitive function important for guiding the on-going or upcoming behavior. A memory-related protein Arc (activity-regulated cytoskeleton-associated protein) is implicated in long-term memory consolidation. Recent evidence further suggests the involvement of hippocampal Arc in spatial WM. The medial prefrontal cortex (mPFC) is a key brain region mediating WM. However, the role of mPFC Arc in WM is still uncertain. To investigate whether mPFC Arc protein is involved in WM performance, delayed non-match to sample (DNMS) T-maze task was performed in rats with or without blocking new synthesis of mPFC Arc. In DNMS task, a 10-s or 30-s delay between the sample run and the choice run was given to evaluate WM performance. To block new Arc protein synthesis during the DNMS task, Arc antisense oligodeoxynucleotides (ODNs) were injected to the bilateral mPFC. The results show that, in rats without surgery for cannula implantation and subsequent intracerebral injection of ODNs, WM was functioning well during the DNMS task with a delay of 10 s but not 30 s, which was accompanied with a significantly increased level of mPFC Arc protein, indicating a possible link between enhanced Arc protein expression and the performance of WM. After preventing the enhancement of mPFC Arc protein expression with Arc antisense ODNs, rat's WM performance was impaired. These findings support enhanced mPFC Arc protein expression playing a role during WM performance.

Histone deacetylase inhibitor attenuates the effects of 27-hydroxycholesterol on the rat brain

Hypercholesterolemia is a risk factor for Alzheimer's disease (AD). Plasma cholesterol does not pass the blood-brain barrier whereas its metabolite 27-hydroxycholesterol (27-OHC) can enter the brain. High 27-OHC in the brain has been suggested to mediate hypercholesterolemia-induced impairments of learning and memory through promoting amyloid-β accumulation and facilitating synaptic disruption. In AD brains, the activity of histone deacetylase (HDAC) is elevated. Treating AD animals with HDAC inhibitors decreases amyloid-β levels and synaptic damages, which leads to memory improvement. Whether HDAC activity is involved in the actions of 27-OHC is still uncertain. In this study, 4 weekly injections of 27-OHC/vehicle were given to rats followed by 3 daily injections of HDAC inhibitor trichostatin (TSA)/vehicle. The results of Morris water maze test reveal that all rats have intact spatial learning ability during the 5-d training phase. However, the behavioral performance during the probe trial was impaired by 27-OHC treatment, which was improved by adding TSA treatments. Furthermore, 27-OHC treatments reduced the hippocampal levels of acetylated histone H3, acetylated α tubulin, insulin-degrading enzyme and postsynaptic protein PSD-95, indicating that 27-OHC treatments may induce enhanced HDAC activity, decreased amyloid-β clearance and synaptic disruption. All reduced levels returned to the basal levels by adding TSA treatments. These findings support our hypothesis that HDAC activity is enhanced following long-term exposure to excess 27-OHC.

The Cholinergic Signaling Responsible for the Expression of a Memory-Related Protein in Primary Rat Cortical Neurons

Cholinergic dysfunction in the brain is closely related to cognitive impairment including memory loss. In addition to the degeneration of basal forebrain cholinergic neurons, deficits in the cholinergic receptor signaling may also play an important role. In the present study, to examine the cholinergic signaling pathways responsible for the induction of a memory-related postsynaptic protein, a cholinergic agonist carbachol was used to induce the expression of activity-regulated cytoskeleton associated protein (Arc) in primary rat cortical neurons. After pretreating neurons with various antagonists or inhibitors, the levels of carbachol-induced Arc protein expression were detected by Western blot analysis. The results show that carbachol induces Arc protein expression mainly through activating M1 acetylcholine receptors and the downstream phospholipase C pathway, which may lead to the activation of the MAPK/ERK signaling pathway. Importantly, carbachol-mediated M2 receptor activation exerts negative effects on Arc protein expression and thus counteracts the enhanced effects of M1 activation. Furthermore, it is suggested for the first time that M1-mediated enhancement of N-methyl-D-aspartate receptor (NMDAR) responses, leading to Ca(2+) entry through NMDARs, contributes to carbachol-induced Arc protein expression. These findings reveal a more complete cholinergic signaling that is responsible for carbachol-induced Arc protein expression, and thus provide more information for developing treatments that can modulate cholinergic signaling and consequently alleviate cognitive impairment. J. Cell. Physiol. 231: 2428-2438, 2016. © 2016 Wiley Periodicals, Inc.

Insulin can induce the expression of a memory-related synaptic protein through facilitating AMPA receptor endocytosis in rat cortical neurons

Learning and memory depend on long-term synaptic plasticity including long-term potentiation (LTP) and depression (LTD). Activity-regulated cytoskeleton-associated protein (Arc) plays versatile roles in synaptic plasticity mainly through inducing F-actin formation, underlying consolidation of LTP, and promoting AMPA receptor (AMPAR) endocytosis, underlying LTD. Insulin can also induce LTD by facilitating the internalization of AMPARs. In neuroblastoma cells, insulin induced a dramatic increase in Arc mRNA and Arc protein levels, which may underlie the memory-enhancing action of insulin. Thus, a hypothesis was made that, in response to insulin, increased AMPAR endocytosis leads to enhanced Arc expression, and vice versa. Primary cultures of neonatal Sprague-Dawley rat cortical neurons were used. Using Western-blot analysis and immunofluorescent staining, our results reveal that inhibiting AMPAR-mediated responses with AMPAR antagonists significantly enhanced whereas blocking AMPAR endocytosis with various reagents significantly prevented insulin (200 nM, 2 h)-induced Arc expression. Furthermore, via surface biotinylation assay, we demonstrate that acute blockade of new Arc synthesis after insulin stimulation using Arc antisense oligodeoxynucleotide prevented insulin-stimulated AMPAR endocytosis. These findings suggest for the first time that an interaction exists between insulin-stimulated AMPAR endocytosis and insulin-induced Arc expression.

Di-(2-ethylhexyl)-phthalate reduces MyoD and myogenin expression and inhibits myogenic differentiation in C2C12 cells

The purpose of this study was to investigate the effects of di-(2-ethylhexyl) phthalate (DEHP) treatment on MyoD and myogenin expression and myotube formation in the murine C2C12 cells. Myogenic differentiation is principally regulated by activities of myogenic regulatory factors, such as MyoD and myogenin, leading the elongation and fusion of mononucleated myoblasts into multinucleated myotubes. In the present study, myogenic differentiation of C2C12 cells was induced by serum deprivation with medium containing vehicle or DEHP (10, 100, 1,000 μg/ml) for 5 days. Using 3-(4,5-dimethylthiazol-2-yl) 2,5-diphenyltetrazolium bromide (MTT) assay clearly demonstrated cell viability was not affected by DEHP at any given dose. At the dose of 1,000 μg/ml DEHP, the elongation of multinucleated myotubes, and the percent of nuclei incorporated into myosin heavy chain (MyHC)-stained myotubes were markedly reduced. In addition, immunoblotting revealed expression of muscle specific marker MyHC, as well as myogenic regulatory factors MyoD and myogenin, were reduced in DEHP-treated myotubes during myogenic differentiation. Taken together, the results showed that DEHP may impair myogenic differentiation through repression of myogenic regulatory factors, such as MyoD and myogenin, resulting in a reduction of MyHC expression. This in vitro study suggests that DEHP may be an environmental risk factor for myogenesis.

Both PKMζ and KIBRA are closely related to reference memory but not working memory in a T-maze task in rats

Protein kinase M zeta (PKMζ) and the kidney and brain protein (KIBRA) play important roles in various forms of memories. However, whether they are involved in performing the T-maze task is still unknown. In this study, the delayed nonmatch-to-sample (DNMS) task in a T-maze was given to rats. The percentage of correct choices denoting the performance accuracy was calculated and the protein levels of PKMζ and KIBRA in rat's prefrontal cortex were measured. The results showed significantly increased performance accuracy after the training phase, which was maintained on the next day in groups with a delay of 10 s but not 30 s, indicating that 30 s is too long for rats to maintain working memory. As for the expressions of PKMζ and KIBRA, significant increases were observed 1 day after the training phase, indicating that the formation of reference memory accompanies an increase in PKMζ and KIBRA. No significant difference was found among groups with various delay intervals, indicating that the expressions of PKMζ and KIBRA exert no effects on the performance of working memory. These results provide the first evidence that KIBRA as well as PKMζ is closely related to reference memory but not working memory in rats.