The triakontatetraneuropeptide (TTN) stimulates thymidine incorporation in rat astrocytes through peripheral-type benzodiazepine receptors

GABAergic regulation of cell proliferation within the adult mouse spinal cord

Manipulation of neural stem cell proliferation and differentiation in the postnatal CNS is receiving significant attention due to therapeutic potential. In the spinal cord, such manipulations may promote repair in conditions such as multiple sclerosis or spinal cord injury, but may also limit excessive cell proliferation contributing to tumours such as ependymomas. We show that when ambient γ-aminobutyric acid (GABA) is increased in vigabatrin-treated or decreased by GAD67 allele haplodeficiency in glutamic acid decarboxylase67-green fluorescent protein (GAD67-GFP) mice of either sex, the numbers of proliferating cells respectively decreased or increased. Thus, intrinsic spinal cord GABA levels are correlated with the extent of cell proliferation, providing important evidence for manipulating these levels. Diazepam binding inhibitor, an endogenous protein that interacts with GABA receptors and its breakdown product, octadecaneuropeptide, which preferentially activates central benzodiazepine (CBR) sites, were highly expressed in spinal cord, especially in ependymal cells surrounding the central canal. Furthermore, animals with reduced CBR activation via treatment with flumazenil or Ro15-4513, or with a G2F77I mutation in the CBR binding site had greater numbers of Ethynyl-2'-deoxyuridine positive cells compared to control, which maintained their stem cell status since the proportion of newly proliferated cells becoming oligodendrocytes or astrocytes was significantly lower. Altering endogenous GABA levels or modulating GABAergic signalling through specific sites on GABA receptors therefore influences NSC proliferation in the adult spinal cord. These findings provide a basis for further study into how GABAergic signalling could be manipulated to enable spinal cord self-regeneration and recovery or limit pathological proliferative activity.

The translocator protein ligand XBD173 improves clinical symptoms and neuropathological markers in the SJL/J mouse model of multiple sclerosis

Multiple sclerosis (MS) is a severe autoimmune disease characterized by inflammatory, demyelinating and neurodegenerative components causing motor, sensory, visual and/or cognitive symptoms. The relapsing-remitting MS affecting 85% of patients is reliably mimicked by the proteolipid-protein (PLP)-induced experimental autoimmune encephalomyelitis (EAE) SJL/J-mouse model. Significant progress was made for MS treatment but the development of effective therapies devoid of severe side-effects remains a great challenge. Here, we combine clinical, behavioral, histopathological, biochemical and molecular approaches to demonstrate that low and well tolerated doses (10-20mg/kg) of TSPO ligand XBD173 (Emapunil) efficiently ameliorate clinical signs and neuropathology of PLP-EAE mice. In addition to the conventional clinical scoring of symptoms, we applied the robust behavioral Catwalk-method to confirm that XBD173 (10mg/kg) increases the maximum contact area parameter at EAE-disease peak, indicating an improvement/recovery of motor functions. Consistently, histopathological studies coupled with microscope-cellSens quantification and RT-qPCR analyzes showed that XBD173 prevented demyelination by restoring normal protein and mRNA levels of myelin basic protein that was significantly repressed in PLP-EAE mice spinal cord and brain. Interestingly, ELISA-based measurement revealed that XBD173 increased allopregnanolone concentrations in PLP-EAE mice spinal and brain tissues. Furthermore, flow cytometry assessment demonstrated that XBD173 therapy decreased serum level of pro-inflammatory cytokines, including interleukin-17A, Interleukin-6 and tumor-necrosis-factor alpha in PLP-EAE mice. As the optimal XBD173 dosing exerting the maximal beneficial action in EAE mice is the lower 10mg/kg dose, the paper opens interesting perspectives for the development of efficient and safe therapies against MS with slight or no side-effects.

TSPO PIGA Ligands Promote Neurosteroidogenesis and Human Astrocyte Well-Being

The steroidogenic 18 kDa translocator protein (TSPO) is an emerging, attractive therapeutic tool for several pathological conditions of the nervous system. Here, 13 high affinity TSPO ligands belonging to our previously described N,N-dialkyl-2-phenylindol-3-ylglyoxylamide (PIGA) class were evaluated for their potential ability to affect the cellular Oxidative Metabolism Activity/Proliferation index, which is used as a measure of astrocyte well-being. The most active PIGA ligands were also assessed for steroidogenic activity in terms of pregnenolone production, and the values were related to the metabolic index in rat and human models. The results showed a positive correlation between the increase in the Oxidative Metabolism Activity/Proliferation index and the pharmacologically induced stimulation of steroidogenesis. The specific involvement of steroid molecules in mediating the metabolic effects of the PIGA ligands was demonstrated using aminoglutethimide, a specific inhibitor of the first step of steroid biosynthesis. The most promising steroidogenic PIGA ligands were the 2-naphthyl derivatives that showed a long residence time to the target, in agreement with our previous data. In conclusion, TSPO ligand-induced neurosteroidogenesis was involved in astrocyte well-being.

TSPO Finds NOX2 in Microglia for Redox Homeostasis

During the past decade, translocator protein 18 kDa (TSPO), previously named peripheral benzodiazepine receptor, has gained a great deal of attention based on its use as a clinical biomarker of neuroinflammation with therapeutic potential. However, there is a paucity of knowledge on the function(s) of TSPO in glial cells. Here, we identify a novel function of TSPO in microglia that is not associated with steroidogenesis. We propose that a TSPO interaction with NADPH oxidase (NOX2) links the generation of reactive oxygen species (ROS) to the induction of an antioxidant response to maintain redox homeostasis. This line of investigation may provide a greater understanding of TSPO glial cell biology, and the knowledge gained may prove beneficial in devising therapeutic strategies.

Astrocytes potentiate GABAergic transmission in the thalamic reticular nucleus via endozepine signaling

Emerging evidence indicates that diazepam-binding inhibitor (DBI) mediates an endogenous benzodiazepine-mimicking (endozepine) effect on synaptic inhibition in the thalamic reticular nucleus (nRT). Here we demonstrate that DBI peptide colocalizes with both astrocytic and neuronal markers in mouse nRT, and investigate the role of astrocytic function in endozepine modulation in this nucleus by testing the effects of the gliotoxin fluorocitrate (FC) on synaptic inhibition and endozepine signaling in the nRT using patch-clamp recordings. FC treatment reduced the effective inhibitory charge of GABAA receptor (GABAAR)-mediated spontaneous inhibitory postsynaptic currents in WT mice, indicating that astrocytes enhance GABAAR responses in the nRT. This effect was abolished by both a point mutation that inhibits classical benzodiazepine binding to GABAARs containing the α3 subunit (predominant in the nRT) and a chromosomal deletion that removes the Dbi gene. Thus, astrocytes are required for positive allosteric modulation via the α3 subunit benzodiazepine-binding site by DBI peptide family endozepines. Outside-out sniffer patches pulled from neurons in the adjacent ventrobasal nucleus, which does not contain endozepines, show a potentiated response to laser photostimulation of caged GABA when placed in the nRT. FC treatment blocked the nRT-dependent potentiation of this response, as did the benzodiazepine site antagonist flumazenil. When sniffer patches were placed in the ventrobasal nucleus, however, subsequent treatment with FC led to potentiation of the uncaged GABA response, suggesting nucleus-specific roles for thalamic astrocytes in regulating inhibition. Taken together, these results suggest that astrocytes are required for endozepine actions in the nRT, and as such can be positive modulators of synaptic inhibition.

Antitumorigenic effect of brain proline rich polypeptide-1 in human chondrosarcoma

Proline rich polypeptide (PRP-1) produced by neurosecretory cells of the hypothalamus is one of the fragments of neurophysin-vasopressin-associated glycoprotein. The primary structure of the neuropeptide PRP-1 isolated from neurosecretory granules of bovine neurohypophysis. We investigated PRP-1 action on chondrosarcoma, the second most common malignancy in bone, which primarily affects the cartilage cells. This deadly disease does not have any effective treatment. Earlier we demonstrated MYC oncogene inactivating effect by 1 lg/ml concentration brain PRP-1 In the present study we observed reduced viable sarcoma JJ012 cell numbers in comparison with control (89% growth inhibition) when treated with low concentrations of PRP-1 (0.5–1 lg/ml). Higher concentrations did not exhibit inhibitory effect. We assume that PRP-1 in low concentration impedes cell cycle progression. The fact that low concentrations of PRP-1 abolished Myc activity prompts to think that the antitumorigenic effect of PRP-1 in low concentrations is mediated through oncogene inactivation.

Diazepam enhances production of diazepam-binding inhibitor (DBI), a negative saliva secretion regulator, localized in rat salivary gland

Peripheral-type benzodiazepine receptor (PBR) and central-type benzodiazepine receptor (CBR) in salivary gland play a role in the inhibitory regulation of salivary secretion in rodents. Diazepam-binding inhibitor (DBI), an endogenous ligand for PBR, produces neurosteroids, which modulate CBR activity. In this study, we investigated the effect of repetitive administration of diazepam (DZP) on salivary secretion and expression of DBI mRNA and peptide. Moreover, mRNA expression of PBR and pituitary adenylate cyclase-activating polypeptide (PACAP), a transcriptional regulator for DBI promoter, was evaluated after repetitive administration of DZP. Repetitive administration, but not single administration, of 0.4 mg/kg DZP caused inhibition of salivary secretion and enhanced expression of DBI, PACAP, and PBR mRNA in rat salivary gland, with an increase in production of DBI peptide. These results suggest that repetitive administration of DZP stimulates DBI production, which may result in an increase in the suppressive effect of DZP on salivary secretion.

Beta-amyloid peptide stimulates endozepine release in cultured rat astrocytes through activation of N-formyl peptide receptors

Astroglial cells synthesize and release endozepines, a family of neuropeptides derived from diazepam-binding inhibitor (DBI). The authors have recently shown that beta-amyloid peptide (Abeta) stimulates DBI gene expression and endozepine release. The purpose of this study was to determine the mechanism of action of Abeta in cultured rat astrocytes. Abeta(25-35) and the N-formyl peptide receptor (FPR) agonist N-formyl-Met-Leu-Phe (fMLF) increased the secretion of endozepines in a dose-dependent manner with EC(50) value of approximately 2 microM. The stimulatory effects of Abeta(25-35) and the FPR agonists fMLF and N-formyl-Met-Met-Met (fMMM) on endozepine release were abrogated by the FPR antagonist N-t-Boc-Phe-Leu-Phe-Leu-Phe. In contrast, Abeta(25-35) increased DBI mRNA expression through a FPR-independent mechanism. Abeta(25-35) induced a transient stimulation of cAMP formation and a sustained activation of polyphosphoinositide turnover. The stimulatory effect of Abeta(25-35) on endozepine release was blocked by the adenylyl cyclase inhibitor somatostatin, the protein kinase A (PKA) inhibitor H89, the phospholipase C inhibitor U73122, the protein kinase C (PKC) inhibitor chelerythrine and the ATP binding cassette transporter blocker glyburide. Taken together, these data demonstrate for the first time that Abeta(25-35) stimulates endozepine release from rat astrocytes through a FPR receptor positively coupled to PKA and PKC.

Astrocytes: adaptive responses to low doses of neurotoxins

This article assesses how astrocytes respond to numerous endogenous agonists and a wide variety of chemical stressors, including well-known neurotoxic agents such as lead and methylmercury, and drugs, within the context of dose-response relationships. In general, astrocytes displayed biphasic dose-response relationships from exposure to neurotoxic agents with temporal and quantitative features similar to hormetic dose responses. While the low-dose stimulatory responses have been viewed as a manifestation of a toxic response, this perspective is being broadly reconsidered and in some cases reinterpreted as being an indicator of an adaptive/protective response. These dose-response patterns are likely to have significant implications in a wide range of conditions affecting normal developmental processes, tumor development, adaptive responses to numerous environmental neurotoxins, and strategies for drug development for the treatment of neurodegenerative conditions.

Translocator protein (18 kDa) ligand PK 11195 induces transient mitochondrial Ca2+ release leading to transepithelial Cl- secretion in HT-29 human colon cancer cells

BACKGROUND INFORMATION

TSPO (translocator protein), known previously as PBR (peripheral-type benzodiazepine receptor), is a 18 kDa protein expressed in the mitochondrial membrane of a variety of tissues. TSPO has been reported to be over-expressed in human colorectal tumours and cancer cell lines, but its function is not well characterized.

RESULTS

We investigated the expression and function of TSPO in the human colon cancer cells HT-29. Immunohistochemical studies revealed that TSPO is localized in mitochondria, and its endogenous ligand, the polypeptide diazepam-binding inhibitor, in the cytosol. Radioligand binding studies using the specific high-affinity drug ligand [(3)H]PK 11195 and membrane fraction demonstrated saturable binding, with K(d) and B(max) values of 13.5+/-1.5 nM and 10.1+/-1.0 pmol/mg respectively. PK 11195 induced a rapid and transient dose-dependent rise in intracellular [Ca(2+)], which was unaffected by extracellular Ca(2+), but was blocked by the PTP (permeability transition pore) inhibitor, cyclosporin A, and by the TSPO partial agonist, flunitrazepam. Using HT-29 clone 19A cell line, which forms cell monolayers, we demonstrated that TSPO ligand stimulated a Ca(2+)-dependent transepithelial Cl(-) secretion. This secretion was inhibited: (i) after removal of extracellular Cl(-); (ii) by apical addition of the Cl(-) channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate]; and (iii) by basolateral addition of the Na(+)-K(+)-2Cl(-) co-transporter inhibitor bumetanide. Furthermore, the intracellular Ca(2+) chelator BAPTA/AM [bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis(acetoxymethyl ester)] and cyclosporin A abolished the rise in PK 11195-induced Cl(-) secretion.

CONCLUSIONS

These findings indicate that TSPO is located in mitochondrial membranes of HT-29 and reveal that its activation induces a rise in cytosolic Ca(2+), leading to the stimulation of Cl(-) secretion.

Beta-amyloid peptides stimulate endozepine biosynthesis in cultured rat astrocytes

Accumulation of beta-amyloid peptide (Abeta), which is a landmark of Alzheimer's disease, may alter astrocyte functions before any visible symptoms of the disease occur. Here, we examined the effects of Abeta on biosynthesis and release of diazepam-binding inhibitor (DBI), a polypeptide primarily expressed by astroglial cells in the CNS. Quantitative RT-PCR and specific radioimmunoassay demonstrated that aggregated Abeta(25-35), at concentrations up to 10(-4) m, induced a dose-dependent increase in DBI mRNA expression and DBI-related peptide release from cultured rat astrocytes. These effects were totally suppressed when aggregation of Abeta(25-35) was prevented by Congo red. Measurement of the number of living cells revealed that Abeta(25-35) induced a trophic rather than a toxic effect on astrocytes. Administration of cycloheximide blocked Abeta(25-35)-induced increase of DBI gene expression and endozepine accumulation in astrocytes, indicating that protein synthesis is required for DBI gene expression. Altogether, the present data suggest that Abeta-induced activation of endozepine biosynthesis and release may contribute to astrocyte proliferation associated with Alzheimer's disease.

Interleukin-8 production induced by the endozepine triakontatetraneuropeptide in human neutrophils: role of calcium and pharmacological investigation of signal transduction pathways

The endozepine triakontatetraneuropeptide (TTN) induces intracellular calcium ([Ca(2+)](i)) changes and is chemotactic for human neutrophils (PMNs). Because interleukin-8 (IL-8) production is Ca(2+) dependent and can be induced by chemotactic stimuli, we have investigated the ability of TTN to induce IL-8 production in PMNs, as well as the signal transduction mechanisms involved. Our results show that TTN increases IL-8 release and IL-8 mRNA expression in a concentration- and time-dependent fashion, and these effects are prevented by the Ca(2+) chelator BAPTA-AM. TTN-induced [Ca(2+)](i) changes and IL-8 mRNA expression are sensitive to pertussis toxin, to the phospholipase C (PLC) inhibitor U73122 (but not to its inactive analogue U73343) and to the protein kinase C (PKC) inhibitor calphostin C. It is therefore suggested that TTN-induced IL-8 production in human PMNs results from a G protein-operated, PLC-activated [Ca(2+)](i) rise, and PKC contributes to this effect. These findings further support the possible role of TTN in the modulation of the inflammatory processes.

Benzodiazepine-induced protein tyrosine nitration in rat astrocytes

Recent studies indicate that ammonia and hypoosmotic astrocyte swelling can induce protein tyrosine nitration (PTN) in astrocytes with potential pathogenetic relevance for hepatic encephalopathy (HE). Because HE episodes are known to be precipitated also by sedatives, the effects of benzodiazepines on PTN in cultured rat astrocytes and rat brain in vivo were studied. In cultured rat astrocytes, diazepam, PK11195, Ro5-4864, and the benzodiazepine binding inhibitor (DBI), which acts on peripheral-type benzodiazepine receptors, induced PTN. Clonazepam, a specific ligand of the central benzodiazepine receptor, failed to induce PTN. Nanomolar concentrations of DBI and PK11195 were sufficient to increase PTN, and diazepam effects were already observed at concentrations of 1 micromol/L. Diazepam-induced PTN was insensitive to NOS inhibition and uric acid but was blunted by MK-801, BAPTA-AM, W13, and catalase, suggesting an involvement of NMDA-receptor activation, elevation of the cytosolic Ca(2+) concentration [Ca(2+)](i), and hydrogen peroxide. Diazepam induced a plateau-like increase in [Ca(2+)](i) and the generation of reactive oxygen intermediates (ROIs), which are both blunted by MK-801 and BAPTA-AM. The expression of functional N-methyl-D-aspartate (NMDA) receptors on cultured rat astrocytes was confirmed by reverse transcriptase polymerase chain reaction, Western blot analysis, immunhistochemistry, and receptor autoradiography. Astroglial PTN is also found in brains from rats challenged with diazepam, indicating the in vivo relevance of the present findings. In conclusion, production of ROIs and increased PTN by benzodiazepines may alter astrocyte function and thereby contribute to the precipitation of HE episodes.

The triakontatetraneuropeptide TTN increases [CA2+]i in rat astrocytes through activation of peripheral-type benzodiazepine receptors

Astrocytes synthesize a series of regulatory peptides called endozepines, which act as endogenous ligands of benzodiazepine receptors. We have recently shown that one of these endozepines, the triakontatetraneuropeptide TTN, stimulates DNA synthesis in astroglial cells. The purpose of the present study was to determine the mechanism of action of TTN on cultured rat astrocytes. Binding of the peripheral-type benzodiazepine receptor ligand [3H]Ro5-4864 to intact astrocytes was displaced by TTN, whereas its C-terminal fragment (TTN[17-34], the octadecaneuropeptide ODN) did not compete for [3H]Ro5-4864 binding. Microfluorimetric measurement of cytosolic calcium concentrations ([Ca2+]i) with the fluorescent probe indo-1 showed that TTN (10(-10) to 10(-6) M) provokes a concentration-dependent increase in [Ca2+]i in cultured astrocytes. Simultaneous administration of TTN (10(-8) M) and Ro5-4864 (10(-5) M) induced an increase in [Ca2+]i similar to that obtained with Ro5-4864 alone. In contrast, the effects of TTN (10(-8) M) and ODN (10(-8) M) on [Ca2+]i were strictly additive. Chelation of extracellular Ca2+ by EGTA (6 mM) or blockage of Ca2+ channels with Ni2+ (2 mM) abrogated the stimulatory effect of TTN. The calcium influx evoked by TTN (10(-7) M) or by Ro5-4864 (10(-5) M) was not affected by the N- and T-type calcium channel blockers omega-conotoxin (10(-6) M) and mibefradil (10(-6) M), but was significantly reduced by the L-type calcium channel blocker nifedipine (10(-7) M). Patch-clamp studies showed that, at negative potentials, TTN (10(-7) M) induced a sustained depolarization. Reduction of the chloride concentration in the extracellular solution shifted the reversal potential from 0 mV to a positive potential. These data show that TTN, acting through peripheral-type benzodiazepine receptors, provokes chloride efflux, which in turn induces calcium influx via L-type calcium channels in rat astrocytes.