Occurrence of the alpha subunits of G proteins in cerebral cortex synaptic membrane and postsynaptic density fractions: modulation of ADP-ribosylation by Ca2+/calmodulin

A network pharmacology study with molecular docking to investigate the possibility of licorice against posttraumatic stress disorder

Post traumatic stress disorder (PTSD) is a mental health condition that has a debilitating effect on a person's quality of life and leads to a high socioeconomic burden. Licorice has been demonstrated to have neuroprotective and antidepressant-like effects, but little is known about its effects for the treatment of PTSD. The present study aimed to explore the potential of licorice for PTSD therapy using a network pharmacology approach with molecular docking studies. The compounds of licorice were obtained from databases with screening by absorption, distribution, metabolism and excretion (ADME) evaluation. Genes associated with compounds or PTSD were obtained from public databases, and the genes overlapping between licorice compounds and PTSD were compared by Venn diagram. A network of medicine-ingredients-targets-disease was constructed, visualized, and analyzed using cytoscape software. Protein-protein interactions, gene ontology, pathway enrichment and molecular docking were performed to evaluate the effect of licorice for the treatment of PTSD. 69 potential compounds were screened after ADME evaluation. A total of 81 compound-related genes and 566 PTSD-related genes were identified in the databases with 27 overlapping genes. Licorice compounds (e.g., medicarpin, 7-methoxy-2-methyl isoflavone, shinpterocarpin, formononetin, licochalcone a) and target proteins (e.g., ESR1, PTGS2, NOS2, and ADRB2) with high degree in the network were involved in G protein-coupled receptor signaling pathways at the postsynaptic/synaptic membrane. Moreover, neuroactive ligand-receptor interactions, calcium signaling, cholinergic synapse, serotonergic synapse and adrenergic signaling in cardiomyocytes may play important roles in the treatment of PTSD by licorice. This study provides molecular evidence of the beneficial effects of licorice for the treatment of PTSD.

RGS inhibition at G(alpha)i2 selectively potentiates 5-HT1A-mediated antidepressant effects

Elevating serotonin (5-HT) levels with selective serotonin reuptake inhibitors (SSRIs) is the most widely used treatment for depression. However, current therapies are ineffective, have delayed benefit, or cause side effects in many patients. Here, we define a mechanism downstream of 5-HT1A receptors that mediates antidepressant-like behavior and is profoundly and selectively enhanced by genetic disruption of regulators of G protein signaling (RGS) activity at G(alpha)i2. Animals rendered insensitive to RGS protein regulation through a mutation in G(alpha)i2 (G184S) exhibited spontaneous antidepressant- and anxiolytic-like behaviors. Mice expressing RGS-insensitive G(alpha)i2 also exhibited increased cortical and hippocampal phosphorylation of glycogen synthase kinase-3beta, a constitutively active proapoptotic kinase that is inhibited through phosphorylation in response to serotonin, SSRIs, and 5-HT1 receptor agonists. Both behavioral and biochemical phenotypes were blocked by treatment with WAY 100635, a 5-HT1A-selective antagonist. RGS-insensitive mice were also 5-10 times more responsive to the antidepressant-like effects of the SSRI fluvoxamine and 5-HT1A-selective agonist 8-hydroxy-2-dipropylaminotetralin. In contrast, the antidepressant potency of agents acting through nonserotonergic mechanisms was unchanged as was 5-HT1A action on body temperature. The findings point to a critical role for endogenous RGS proteins to suppress the antidepressant-like effects of 5-HT1A receptor activation. By selectively enhancing the beneficial effects of serotonin, inhibition of RGS proteins represents a therapeutic approach for the treatment of mood disorders.

Hyperdopaminergic tone erodes prefrontal long-term potential via a D2 receptor-operated protein phosphatase gate

Dopamine (DA) plays crucial roles in the cognitive functioning of the prefrontal cortex (PFC), which, to a large degree, depends on lasting neural traces formed in prefrontal networks. The establishment of these permanent traces requires changes in cortical synaptic efficacy. DA, via the D(1)-class receptors, is thought to gate or facilitate synaptic plasticity in the PFC, with little role recognized for the D(2)-class receptors. Here we show that, when significantly elevated, DA erodes, rather than facilitates, the induction of long-term potentiation (LTP) in the PFC by acting at the far less abundant cortical D(2)-class receptors through a dominant coupling to the protein phosphatase 1 (PP1) activity in postsynaptic neurons. In mice with persistently elevated extracellular DA, resulting from inactivation of the DA transporter (DAT) gene, LTP in layer V PFC pyramidal neurons cannot be established, regardless of induction protocols. Acute increase of dopaminergic transmission by DAT blockers or overstimulation of D(2) receptors in normal mice have similar LTP shutoff effects. LTP in mutant mice can be rescued by a single in vivo administration of D(2)-class antagonists. Suppression of postsynaptic PP1 mimics and occludes the D(2)-mediated rescue of LTP in mutant mice and prevents the acute erosion of LTP by D(2) agonists in normal mice. Our studies reveal a mechanistically unique heterosynaptic PP1 gate that is constitutively driven by background DA to influence LTP induction. By blocking prefrontal synaptic plasticity, excessive DA may prevent storage of lasting memory traces in PFC networks and impair executive functions.

Characterization of mRNA species that are associated with postsynaptic density fraction by gene chip microarray analysis

We previously reported the partial identification by random sequencing of mRNA species that are associated with the postsynaptic density (PSD) fraction prepared from the rat forebrain [Tian et al., 1999. Mol. Brain Res. 72, 147-157]. We report here further characterization by gene chip analysis of the PSD fraction-associated mRNAs, which were prepared in the presence of RNase inhibitor. We found that mRNAs encoding various postsynaptic proteins, such as channels, receptors for neurotransmitters and neuromodulators, proteins involved in signaling, scaffold and adaptor proteins and cytoskeletal proteins, were highly concentrated in the PSD fraction, whereas those encoding housekeeping proteins, such as enzymes in the glycolytic pathway, were not. We extracted approximately 1900 mRNA species that were highly concentrated in the PSD fraction. mRNAs related to certain neuronal diseases were also enriched in the PSD fraction. We also constructed a cDNA library using the PSD fraction-associated mRNAs as templates, and identified 1152 randomly selected clones by sequencing. Our data suggested that the PSD fraction-associated mRNAs are a very useful resource, in which a number of as yet uncharacterized mRNAs are concentrated. Identification and functional characterization of them are essential for complete understanding of synaptic function.

Molecular constituents and phosphorylation-dependent regulation of the post-synaptic density

The post-synaptic density (PSD) contains receptors with associated signaling- and scaffolding-proteins that organize signal-transduction pathways near the post-synaptic membrane. The PSD plays an important role in synaptic plasticity, and protein phosphorylation is critical to the regulation of PSD function, including learning and memory. Recently, studies have investigated the protein constituents of the PSD and substrate proteins for various protein kinases by proteomic analysis. The present review focuses on the molecular properties of PSD proteins, and substrates of protein kinases and their regulation by phosphorylation in order to understand the role of PSD in synaptic plasticity.

Pathophysiological implications of the structural organization of the excitatory synapse

The glutamatergic synapse is the key structure in the development of activity-dependent synaptic plasticity in the central nervous system. The analysis of the complex biochemical mechanisms at the basis of the long-term changes in synaptic efficacy have received a tremendous impulse by the observation that the post-synaptic constituents of the synapse can be separated and purified through a simple procedure involving detergent treatment of synaptosomes and differential centrifugation. In this fraction, called post-synaptic density (PSD), the functional interactions of its constituents are preserved. The various subunits of ionotropic glutamate receptors are held in register with the presynaptic active zone through their interaction with linker proteins. N-methyl-D-aspartate (NMDA) subunits NR2A and NR2B, bind to the PSD protein called PSD-95, which in turn binds neuroligins, providing a handle for interacting with neurexin, located in the plasma membrane at the presynaptic active zone. Additional clustering of NMDA receptors is provided through the binding of NRI subunits to the cytoskeletal protein alpha-actinin-2. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and kainate receptors are other important constituents of PSDs and bind to different anchoring proteins. Phosphorylation processes have long been known to modulate NMDA receptor functional activity: the finding that several protein kinases, particularly Ca2+/Calmodulin-dependent protein kinase II and protein tyrosine kinases of the src family, are major constituents of PSDs has allowed to demonstrate that these enzymes are localized in a strategic position of the glutamatergic synapse, so that their activation provides a means for NMDA receptor function regulation upon its activation. The relevance of these mechanisms has been demonstrated in experimental models of pathologies involving deficits in synaptic plasticity, such as in streptozotocin-induced diabetes and in an animal model of prenatal induced ablation of hippocampal neurons. Both animal models display disturbances in long-term potentiation and cognitive deficits, thus providing in vivo models to study pathology related changes in both the structure and the function of the excitatory synapse.

The Ras Guanine nucleotide Exchange Factor CDC25Mm is present at the synaptic junction

CDC25Mm, a mouse Ras-Guanine nucleotide Exchange Factor, is specifically expressed as a product of 140 kDa (p140) in the postnatal and adult brain. Immunohistochemical analysis indicates that it is present throughout the brain particularly concentrated in discrete punctate structures. Subcellular fractionation of the mouse brain shows that p140 is present in synaptosomes but not in highly purified synaptic vesicles. Moreover, isolated postsynaptic densities (PSDs) are largely enriched in CDC25Mm. This protein can be phosphorylated by calcium/calmodulin kinase II, the most abundant protein in PSDs. Altogether these results suggest that CDC25Mm is present at synaptic junctions and that it may be involved in synaptic signal transduction leading to Ras activation.

Involvement of a heterotrimeric G protein alpha subunit in tight junction biogenesis

The tight junction (TJ) of polarized epithelial cells is critical for maintaining an impermeant barrier and epithelial cell polarity. The signaling events important for TJ assembly require regulated calcium stores and protein kinase C (PKC), but the earliest signaling events in the cascade have not been well defined. We now show that Galphai2 in Madin Darby canine kidney (MDCK) cells localizes to a region overlapping with the TJ. To further analyze the localization of Galpha subunits in epithelial cells, rat Galphao, Q205Lalphao (Galphao "activated" by point mutation) and plasmid without insert (PC) were transfected into MDCK cells and localized by immunofluorescence and confocal microscopy. Similar to endogenous Galphai2, Galphao-MDCK cells localize Galphao, (84% similar to Galphai2) in the subapical region overlapping with ZO-1 (zona occludens-I), a key component of the TJ. PC-MDCK cells have no detectable Galphao. In Galphao-MDCK cells, a physical association of Galphao with components of the TJ was detectable by immunoprecipitation of ZO-1. Immunoprecipitates of ZO-1 from Galphao-MDCK cells consistently coprecipitated Galphao. Constitutively active Q205LGalphao localized to the subapical lateral membrane similar to wild-type Galphao. To determine if constitutively activated Galpha subunits can affect TJ biogenesis, the formation of tight junctions in PC, Galphao, and Q205Lalphao-MDCK cells was followed by measurement of transepithelial resistance (TER) during the Ca2+ switch, a model widely used to study mechanisms of junctional assembly. Baseline and post Ca2+ switch TER values did not differ among the cell lines. However, constitutively activated Q205Lalphao-MDCK cells developed TER significantly faster than PC and Galphao cells in the early phase (0-4 h) (54 +/- 4 versus 23 +/- 3 (PC); 12 +/- 1 (Galphao) Omega.m2/h) and late phase (4-h peak) (117 +/- 10 versus 45 +/- 5 (PC); 66 +/- 7 (Galphao) Omega.m2/h) after Ca2+ switch. Peak TER values were significantly higher in Q205Lalphao-MDCK cells (1168 +/- 107 versus 437 +/- 37 (PC); 548 +/- 54 (Galphao) Omega.cm2). These results indicate that Galphao and Q205Lalphao expressed in MDCK cells are localized near the junctional complex, associate with at least one TJ protein, and that activated Galphao accelerates TJ biogenesis without significantly affecting the maintenance of the TJ. Together, these results suggest an important role for heterotrimeric G proteins in TJ assembly.