miR-154-5p Is a Novel Endogenous Ligand for TLR7 Inducing Microglial Activation and Neuronal Injury.

Toll-like receptors (TLRs) are a collection of pattern recognition sensors that form a first line of defence by detecting pathogen- or damage-associated molecular patterns and initiating an inflammatory response. TLR activation in microglia, the major immune cells in the brain, can trigger the release of inflammatory molecules, which may contribute to various CNS diseases including Alzheimer's disease. Recently, some microRNAs were shown to serve as signalling molecules for TLRs. Here, we present miR-154-5p as a novel TLR7 ligand. Exposing microglia to miR-154-5p results in cytokine release and alters expression of the TLR signalling pathway dependent on TLR7. Additionally, miR-154-5p causes neuronal injury in enriched cortical neuron cultures and additive toxicity in the presence of microglia. Finally, intrathecal injection of miR-154-5p into mice leads to neuronal injury and accumulation of microglia in the cerebral cortex dependent on TLR7 expression. In conclusion, this study establishes miR-154-5p as a direct activator of TLR7 that can cause neuroinflammation and neuronal injury, which may contribute to CNS disease.

MicroRNA-100-5p and microRNA-298-5p released from apoptotic cortical neurons are endogenous Toll-like receptor 7/8 ligands that contribute to neurodegeneration.

BACKGROUND: MicroRNA (miRNA) expression in the brain is altered in neurodegenerative diseases. Recent studies demonstrated that selected miRNAs conventionally regulating gene expression at the post-transcriptional level can act extracellularly as signaling molecules. The identity of miRNA species serving as membrane receptor ligands involved in neuronal apoptosis in the central nervous system (CNS), as well as the miRNAs' sequence and structure required for this mode of action remained largely unresolved. METHODS: Using a microarray-based screening approach we analyzed apoptotic cortical neurons of C56BL/6 mice and their supernatant with respect to alterations in miRNA expression/presence. HEK-Blue Toll-like receptor (TLR) 7/8 reporter cells, primary microglia and macrophages derived from human and mouse were employed to test the potential of the identified miRNAs released from apoptotic neurons to serve as signaling molecules for the RNA-sensing receptors. Biophysical and bioinformatical approaches, as well as immunoassays and sequential microscopy were used to analyze the interaction between candidate miRNA and TLR. Immunocytochemical and -histochemical analyses of murine CNS cultures and adult mice intrathecally injected with miRNAs, respectively, were performed to evaluate the impact of miRNA-induced TLR activation on neuronal survival and microglial activation. RESULTS: We identified a specific pattern of miRNAs released from apoptotic cortical neurons that activate TLR7 and/or TLR8, depending on sequence and species. Exposure of microglia and macrophages to certain miRNA classes released from apoptotic neurons resulted in the sequence-specific production of distinct cytokines/chemokines and increased phagocytic activity. Out of those miRNAs miR-100-5p and miR-298-5p, which have consistently been linked to neurodegenerative diseases, entered microglia, located to their endosomes, and directly bound to human TLR8. The miRNA-TLR interaction required novel sequence features, but no specific structure formation of mature miRNA. As a consequence of miR-100-5p- and miR-298-5p-induced TLR activation, cortical neurons underwent cell-autonomous apoptosis. Presence of miR-100-5p and miR-298-5p in cerebrospinal fluid led to neurodegeneration and microglial accumulation in the murine cerebral cortex through TLR7 signaling. CONCLUSION: Our data demonstrate that specific miRNAs are released from apoptotic cortical neurons, serve as endogenous TLR7/8 ligands, and thereby trigger further neuronal apoptosis in the CNS. Our findings underline the recently discovered role of miRNAs as extracellular signaling molecules, particularly in the context of neurodegeneration.

Reexamination of N-terminal domains of syntaxin-1 in vesicle fusion from central murine synapses.

Syntaxin-1 (STX1) and Munc18-1 are two requisite components of synaptic vesicular release machinery, so much so synaptic transmission cannot proceed in their absence. They form a tight complex through two major binding modes: through STX1's N-peptide and through STX1's closed conformation driven by its Habc- domain. However, physiological roles of these two reportedly different binding modes in synapses are still controversial. Here we characterized the roles of STX1's N-peptide, Habc-domain, and open conformation with and without N-peptide deletion using our STX1-null mouse model system and exogenous reintroduction of STX1A mutants. We show, on the contrary to the general view, that the Habc-domain is absolutely required and N-peptide is dispensable for synaptic transmission. However, STX1A's N-peptide plays a regulatory role, particularly in the Ca2+-sensitivity and the short-term plasticity of vesicular release, whereas STX1's open conformation governs the vesicle fusogenicity. Strikingly, we also show neurotransmitter release still proceeds when the two interaction modes between STX1A and Munc18-1 are presumably intervened, necessitating a refinement of the conceptualization of STX1A-Munc18-1 interaction.

Activation of Toll-like receptor 5 in microglia modulates their function and triggers neuronal injury.

Microglia are the primary immune-competent cells of the central nervous system (CNS) and sense both pathogen- and host-derived factors through several receptor systems including the Toll-like receptor (TLR) family. Although TLR5 has previously been implicated in different CNS disorders including neurodegenerative diseases, its mode of action in the brain remained largely unexplored. We sought to determine the expression and functional consequences of TLR5 activation in the CNS. Quantitative real-time PCR and immunocytochemical analysis revealed that microglia is the major CNS cell type that constitutively expresses TLR5. Using Tlr5-/- mice and inhibitory TLR5 antibody we found that activation of TLR5 in microglial cells by its agonist flagellin, a principal protein component of bacterial flagella, triggers their release of distinct inflammatory molecules, regulates chemotaxis, and increases their phagocytic activity. Furthermore, while TLR5 activation does not affect tumor growth in an ex vivo GL261 glioma mouse model, it triggers microglial accumulation and neuronal apoptosis in the cerebral cortex in vivo. TLR5-mediated microglial function involves the PI3K/Akt/mammalian target of rapamycin complex 1 (mTORC1) pathway, as specific inhibitors of this signaling pathway abolish microglial activation. Taken together, our findings establish TLR5 as a modulator of microglial function and indicate its contribution to inflammatory and injurious processes in the CNS.

Antidepressant-like effect of Albizia zygia root extract in murine models.

Background Albizia zygia (DC.) J.F. Macbr. (Leguminosae) has been used to treat mental disorders in traditional African medicine. Nonetheless, there is limited scientific evidence to justify its present use. The aim of this study was to evaluate the antidepressant activity of the hydroethanolic extract of A. zygia roots (AZE) in murine models. Methods AZE was evaluated in the tail suspension test, forced swim test, and the repeated open-space swim test of depression. In order to elucidate the mechanisms of action, the activity of AZE was re-evaluated after treating mice with selective inhibitors of monoamine biosynthesis. The potential of AZE to influence spontaneous locomotion was also examined. Results AZE (100-1000 mg/kg, p.o.) reduced the immobility time of mice in the tail suspension and forced swim tests (at least p < 0.05). In the repeated open-space swim test, AZE reduced the immobility time (at least p < 0.05) while concomitantly increasing the distance swam by mice (p < 0.01). However, the antidepressant-like activity of AZE was attenuated by α-methyl-para-tyrosine and reserpine (p < 0.0001) but not para-chlorophenylalanine. Conclusions The results of this study indicate that AZE possesses antidepressant-like properties and support the traditional use of AZE for the treatment of depression.

Albizia zygia root extract exhibits antipsychotic-like properties in murine models of schizophrenia.

BACKGROUND: The root extract of Albizia zygia (DC.) J.F. Macbr. (Leguminosae) is used to manage mental disorders in African traditional medicine. However, its value, particularly, against negative and cognitive symptoms of schizophrenia have not been evaluated. AIM: The aim of this study was to evaluate the antipsychotic properties of the hydroethanolic root extract of Albizia zygia (AZE) against positive, negative and cognitive symptoms of schizophrenia in animal models. MATERIALS AND METHODS: The effects of AZE (30-300 mg kg-1) were evaluated against apomorphine-induced cage climbing as well as ketamine -induced hyperlocomotion, -enhanced immobility, -impaired social interaction and novel object recognition. The propensity of AZE to induce catalepsy and to attenuate haloperidol-induced catalepsy were also investigated. RESULTS: AZE 30-300 mg kg-1 significantly reduced apomorphine-induced climbing behaviour as well as ketamine-induced hyperlocomotion, immobility and object recognition deficits (at least P < 0.05). Moreover, the extract showed no cataleptic effect but significantly inhibited haloperidol-induced catalepsy at a dose of 30 mg kg-1 (P < 0.05). CONCLUSION: The root extract of Albizia zygia exhibited an antipsychotic-like activity in mice with potential to alleviate positive, negative and cognitive symptoms of schizophrenia.