Regulation of aberrant proteasome activity re-establishes plasticity and long-term memory in an animal model of Alzheimer's disease.

Reduced retrograde memory performance at the cognitive level and aggregation/deposition of amyloid beta (Aß) in the brain at the cellular level are some of the hallmarks of Alzheimer's Disease (AD). A molecular system that participates in the removal of proteins with an altered conformation is the Ubiquitin-Proteasome System (UPS). Impairments of the UPS in wild-type (WT) mice lead to defective clearance of Aß and prevent long-term plasticity of synaptic transmission. Here we show data whereby in contrast to WT mice, the inhibition of proteasome-mediated protein degradation in an animal model of AD by MG132 or lactacystin restores impaired activity-dependent synaptic plasticity and its associative interaction, synaptic tagging and capture (STC) in vitro, as well as associative long-term memory in vivo. This augmentation of synaptic plasticity and memory is mediated by the mTOR pathway and protein synthesis. Our data offer novel insights into the rebalancing of proteins relevant for synaptic plasticity which are regulated by UPS in AD-like animal models. In addition, the data provide evidence that proteasome inhibitors might be effective in reinstating synaptic plasticity and memory performance in AD, and therefore offer a new potential therapeutic option for AD treatment.

Substance P induces plasticity and synaptic tagging/capture in rat hippocampal area CA2.

The hippocampal area Cornu Ammonis (CA) CA2 is important for social interaction and is innervated by Substance P (SP)-expressing supramammillary (SuM) nucleus neurons. SP exerts neuromodulatory effects on pain processing and central synaptic transmission. Here we provide evidence that SP can induce a slowly developing NMDA receptor- and protein synthesis-dependent potentiation of synaptic transmission that can be induced not only at entorhinal cortical (EC)-CA2 synapses but also at long-term potentiation (LTP)-resistant Schaffer collateral (SC)-CA2 synapses. In addition, SP-induced potentiation of SC-CA2 synapses transforms a short-term potentiation of EC-CA2 synaptic transmission into LTP, consistent with the synaptic tagging and capture hypothesis. Interestingly, this SP-induced potentiation and associative interaction between the EC and SC inputs of CA2 neurons is independent of the GABAergic system. In addition, CaMKIV and PKMζ play a critical role in the SP-induced effects on SC-CA2 and EC-CA2 synapses. Thus, afferents from SuM neurons are ideally situated to prime CA2 synapses for the formation of long-lasting plasticity and associativity.

Tumor-mediated microbiota alteration impairs synaptic tagging/capture in the hippocampal CA1 area via IL-1ß production.

Cancer patients often experience impairments in cognitive function. However, the evidence for tumor-mediated neurological impairment and detailed mechanisms are still lacking. Gut microbiota has been demonstrated to be involved in the immune system homeostasis and brain functions. Here we find that hepatocellular carcinoma (HCC) growth alters the gut microbiota and impedes the cognitive functions. The synaptic tagging and capture (STC), an associative cellular mechanism for the formation of associative memory, is impaired in the tumor-bearing mice. STC expression is rescued after microbiota sterilization. Transplantation of microbiota from HCC tumor-bearing mice induces similar STC impairment in wide type mice. Mechanistic study reveals that HCC growth significantly elevates the serum and hippocampus IL-1ß levels. IL-1ß depletion in the HCC tumor-bearing mice restores the STC. Taken together, these results demonstrate that gut microbiota plays a crucial role in mediating the tumor-induced impairment of the cognitive function via upregulating IL-1ß production.

Expression of sphingosine kinase 1 in amoeboid microglial cells in the corpus callosum of postnatal rats.

Sphingosine kinase 1 (SphK1), a key enzyme responsible for phosphorylating sphingosine into sphingosine-1-phosphate (S1P) has been shown to be expressed in monocytes and monocyte-derived peripheral macrophages. This study demonstrates SphK1 immunoexpression in amoeboid microglial cells (AMC), a nascent monocyte-derived brain macrophage in the corpus callosum of developing rat brain. SphK1 immunofluorescence expression, which appeared to be weak in AMC in normal brain, was markedly induced by lipopolysaccharide (LPS) or hypoxia treatment. Western blot analysis also showed increased expression level of SphK1 in the corpus callosum rich in AMC after LPS treatment. Detection of SphK1 mRNA and its upregulation after LPS treatment was confirmed in primary culture AMC by RT-PCR. Administration of N, N-dimethylsphingosine (DMS), a specific inhibitor of SphK1, effectively reduced upregulated SphK1 immunoexpression in AMC both in vivo and in vitro. This was corroborated by western blot which showed a decrease in SphK1 protein level of callosal tissue with DMS pretreatment. Remarkably, LPS-induced upregulation of the transcription factor NFκB was suppressed by DMS. We conclude that SphK1 expression in AMC may be linked to regulation of proinflammatory cytokines via an NFκB signaling pathway.

MicroRNA-134-5p inhibition rescues long-term plasticity and synaptic tagging/capture in an Aß(1-42)-induced model of Alzheimer's disease.

Progressive memory loss is one of the most common characteristics of Alzheimer's disease (AD), which has been shown to be caused by several factors including accumulation of amyloid ß peptide (Aß) plaques and neurofibrillary tangles. Synaptic plasticity and associative plasticity, the cellular basis of memory, are impaired in AD. Recent studies suggest a functional relevance of microRNAs (miRNAs) in regulating plasticity changes in AD, as their differential expressions were reported in many AD brain regions. However, the specific role of these miRNAs in AD has not been elucidated. We have reported earlier that late long-term potentiation (late LTP) and its associative mechanisms such as synaptic tagging and capture (STC) were impaired in Aß (1-42)-induced AD condition. This study demonstrates that expression of miR-134-5p, a brain-specific miRNA is upregulated in Aß (1-42)-treated AD hippocampus. Interestingly, the loss of function of miR-134-5p restored late LTP and STC in AD. In AD brains, inhibition of miR-134-5p elevated the expression of plasticity-related proteins (PRPs), cAMP-response-element binding protein (CREB-1) and brain-derived neurotrophic factor (BDNF), which are otherwise downregulated in AD condition. The results provide the first evidence that the miR-134-mediated post-transcriptional regulation of CREB-1 and BDNF is an important molecular mechanism underlying the plasticity deficit in AD; thus demonstrating the critical role of miR-134-5p as a potential therapeutic target for restoring plasticity in AD condition.

Group III metabotropic glutamate receptors gate long-term potentiation and synaptic tagging/capture in rat hippocampal area CA2.

Metabotropic glutamate receptors (mGluRs) play an important role in synaptic plasticity and memory and are largely classified based on amino acid sequence homology and pharmacological properties. Among group III metabotropic glutamate receptors, mGluR7 and mGluR4 show high relative expression in the rat hippocampal area CA2. Group III metabotropic glutamate receptors are known to down-regulate cAMP-dependent signaling pathways via the activation of Gi/o proteins. Here, we provide evidence that inhibition of group III mGluRs by specific antagonists permits an NMDA receptor- and protein synthesis-dependent long-lasting synaptic potentiation in the apparently long-term potentiation (LTP)-resistant Schaffer collateral (SC)-CA2 synapses. Moreover, long-lasting potentiation of these synapses transforms a transient synaptic potentiation of the entorhinal cortical (EC)-CA2 synapses into a stable long-lasting LTP, in accordance with the synaptic tagging/capture hypothesis (STC). Furthermore, this study also sheds light on the role of ERK/MAPK protein signaling and the downregulation of STEP protein in the group III mGluR inhibition-mediated plasticity in the hippocampal CA2 region, identifying them as critical molecular players. Thus, the regulation of group III mGluRs provides a conducive environment for the SC-CA2 synapses to respond to events that could lead to activity-dependent synaptic plasticity.

Runx1t1 (Runt-related transcription factor 1; translocated to, 1) epigenetically regulates the proliferation and nitric oxide production of microglia.

BACKGROUND: Microglia, the resident immune cells of the brain, undergo rapid proliferation and produce several proinflammatory molecules and nitric oxide (NO) when activated in neuropathological conditions. Runx1t1 (Runt-related transcription factor 1, translocated to 1) has been implicated in recruiting histone deacetylases (HDACs) for transcriptional repression, thereby regulating cell proliferation. In the present study, Runx1t1 expression was shown to localize in amoeboid microglial cells of the postnatal rat brain, being hardly detectable in ramified microglia of the adult brain. Moreover, a marked expression of Runx1t1was induced and translocated to nuclei in activated microglia in vitro and in vivo. In view of these findings, it was hypothesized that Runx1t1 regulates microglial functions during development and in neuropathological conditions. METHODS AND FINDINGS: siRNA-mediated knockdown of Runx1t1 significantly decreased the expression level of cell cycle-related gene, cyclin-dependent kinase 4 (Cdk4) and proliferation index in activated BV2 microglia. It was also shown that HDAC inhibitor (HDACi) treatment mimics the effects of Runx1t1 knockdown on microglial proliferation, confirming that microglial proliferation is associated with Runx1t1 expression and HDACs activity. Further, Runx1t1 and HDACs were shown to promote neurotoxic effect of microglia by repressing expression of LAT2, L-aminoacid transporter-2 (cationic amino acid transporter, y+ system), which normally inhibits NO production. This was confirmed by chromatin immunoprecipitation (ChIP) assay, which revealed that Runx1t1 binds to the promoter region of LAT2 and this binding increased upon microglial activation. However, the enhanced binding of Runx1t1 to the LAT2 promoter could not repress the LAT2 expression when the BV2 microglia cells were treated with HDACi, indicating that Runx1t1 requires HDACs to transcriptionally repress the expression of LAT2. CONCLUSION/INTERPRETATION: In conclusion, it is suggested that Runx1t1 controls proliferation and the neurotoxic effect of microglia by epigenetically regulating Cdk4 and LAT2 via its interaction with HDACs.

Costunolide inhibits proinflammatory cytokines and iNOS in activated murine BV2 microglia.

Costunolide, a sesquiterpene lactone present in Costus speciosus root exerts a variety of pharmacological activity but its effects on neuroinflammation have not been studied. Microglia, the resident phagocytic cells in the central nervous system respond to neuroinflammation and their overwhelming response in turn aggravate brain damage during infection, ischemia and neurodegenerative diseases. In this study, we report the effect of Costunolide on the production of proinflammatory mediators and mechanisms involved in BV2 microglial cells stimulated with LPS. Costunolide attenuated the expression of tumour necrosis factor-alpha, interleukin-1,6, inducible nitric oxide synthase, monocyte chemotactic protein 1 and cyclooxygenase 2 in activated microglia. This Costunolide-mediated inhibition was correspondent with the inhibition of NFkappaB activation. It has been further shown that Costunolide suppressed MAPK pathway activation by inducing MKP-1 production. Collectively our results suggest that Costunolide shows an ability to inhibit expression of multiple neuroinflammatory mediators and this is attributable to the compounds inhibition of NFkappaB and MAPK activation. This novel role of Costunolide upon investigation may aid in developing better therapeutic strategies for treatment of neuroinflammatory diseases.