Activation of VIP/PACAP type 2 receptor by the peptide histidine isoleucine in astrocytes influences GLAST-mediated glutamate uptake

Pharmacological evidence for the concept of spare glutamate transporters

Through the efficient clearance of extracellular glutamate, high affinity astrocytic glutamate transporters constantly shape excitatory neurotransmission in terms of duration and spreading. Even though the glutamate transporter GLT-1 (also known as EAAT2/SLC1A2) is amongst the most abundant proteins in the mammalian brain, its density and activity are tightly regulated. In order to study the influence of changes in the expression of GLT-1 on glutamate uptake capacity, we have developed a model in HEK cells where the density of the transporter can be manipulated thanks to a tetracycline-inducible promoter. Exposing the cells to doxycycline concentration-dependently increased GLT-1 expression and substrate uptake velocity. However, beyond a certain level of induction, increasing the density of transporters at the cell surface failed to increase the maximal uptake. This suggested the progressive generation of a pool of spare transporters, a hypothesis that was further validated using the selective GLT-1 blocker WAY-213613 of which potency was influenced by the density of the transporters. The curve showing inhibition of uptake by increasing concentrations of WAY-213613 was indeed progressively rightward shifted when tested in cells where the transporter density was robustly induced. As largely documented in the context of cell-surface receptors, the existence of 'spare' glutamate transporters in the nervous tissue and particularly in astrocytes could impact on the consequences of physiological or pathological regulation of these transporters.

β₂-adrenergic agonists modulate TNF-α induced astrocytic inflammatory gene expression and brain inflammatory cell populations

BACKGROUND

The NF-κB signaling pathway orchestrates many of the intricate aspects of neuroinflammation. Astrocytic β₂-adrenergic receptors have emerged as potential regulators in central nervous system inflammation and are potential targets for pharmacological modulation. The aim of this study was to elucidate the crosstalk between astrocytic β₂-adrenergic receptors and the TNF-α induced inflammatory gene program.

METHODS

Proinflammatory conditions were generated by the administration of TNF-α. Genes that are susceptible to astrocytic crosstalk between β₂-adrenergic receptors (stimulated by clenbuterol) and TNF-α were identified by qPCR-macroarray-based gene expression analysis in a human 1321 N1 astrocytoma cell line. Transcriptional patterns of the identified genes in vitro were validated by RT-PCR on the 1321 N1 cell line as well as on primary rat astrocytes. In vivo expression patterns were examined by intracerebroventricular administration of clenbuterol and/or TNF-α in rats. To examine the impact on the inflammatory cell content of the brain we performed extensive FACS analysis of rat brain immune cells after intracerebroventricular clenbuterol and/or TNF-α administration.

RESULTS

Parallel transcriptional patterns in vivo and in vitro confirmed the relevance of astrocytic β₂-adrenergic receptors as modulators of brain inflammatory responses. Importantly, we observed pronounced effects of β2-adrenergic receptor agonists and TNF-α on IL-6, CXCL2, CXCL3, VCAM1, and ICAM1 expression, suggesting a role in inflammatory brain cell homeostasis. Extensive FACS-analysis of inflammatory cell content in the brain demonstrated that clenbuterol/TNF-α co-administration skewed the T cell population towards a double negative phenotype and induced a shift in the myeloid brain cell population towards a neutrophilic predominance.

CONCLUSIONS

Our results show that astrocytic β₂-adrenergic receptors are potent regulators of astrocytic TNF-α-activated genes in vitro and in vivo, and ultimately modulate the molecular network involved in the homeostasis of inflammatory cells in the central nervous system. Astrocytic β₂-adrenergic receptors and their downstream signaling pathway may serve as potential targets to modulate neuroinflammatory responses.

Differential regulation of vasoactive intestinal peptide (VIP) in the dentate gyrus and hippocampus via the NO-cGMP pathway following kainic acid-induced seizure in the rat

We have previously shown that kainic acid (KA) increases nitric oxide (NO) synthase (NOS) production in the rat dentate gyrus (DG) and hippocampus (CA3), and NOS inhibition [(by N(G)-nitro-L-arginine methylester (L-NAME)] modulates the vasoactive intestinal peptide (VIP)-responsive gene, activity-dependent neuroprotective protein, and alters neuro- and astrogliogenesis (Cosgrave et al. in Neurobiol Dis 30(3):281-292 2008, J Mol Neurosci 39(1-2):9-21, 2009, 2010). In the present study, using the same model we demonstrate that VIP synthesis is differentially regulated by the NO-cyclic guanosine monophosphate (cGMP) pathway in the DG and CA3 at 3 h and 3 days post-KA. At 3 h post-KA: In L-NAME+KA/7-nitroindazole (7-NI)+KA, stratum granulosum (SG) and subgranular zone (SGZ) cells were intensely stained for VIP when compared with L-NAME/7-NI/KA alone. Soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, blocks cGMP production), suppressed astrocytic activation (glial fibrillary acidic protein) but other cell types were VIP(+); however, ODQ+KA suppressed overall VIP synthesis in the DG. At 3 days post-KA: In L-NAME+KA/7-NI+KA, SGZ and SG cells continued to express VIP, while in the KA alone, only SGZ cells were VIP(+). ODQ increased VIP(+) cells in the SG, and in contrast to 3 h, VIP-containing nNOS(+) cells increased in ODQ+KA when compared to vehicle+KA. In the hippocampus, 7-NI/ODQ had no effect on VIP at 3 h/3 days, while L-NAME+KA at 3 days increased VIP(+) cells, but reduced VIP-like immunoreactivity in astrocytes. These results suggest that the NO-cGMP pathway differentially regulates VIP in the DG and hippocampus during seizure.

Vasoactive intestinal peptide acts via multiple signal pathways to regulate hippocampal NMDA receptors and synaptic transmission

Vasoactive intestinal peptide (VIP) is a 28-amino acid peptide, which belongs to a superfamily of structurally related peptide hormones including pituitary adenylate cyclase-activating polypeptide (PACAP). Although several studies have identified the involvement of PACAP in learning and memory, little work has been done to investigate such a role for VIP. At least three receptors for VIP have been identified including the PACAP receptor (PAC1-R) and the two VIP receptors (VPAC receptors). VIP can activate the PAC1-R only if it is used at relatively high concentrations (e.g., 100 nM); however, at lower concentrations (e.g., 1 nM) it is selective for the VPAC receptors. Our lab has showed that PAC1-R activation signals through PKC/CAKbeta/Src pathway to regulate NMDA receptors; however, there is little known about the potential regulation of NMDA receptors by VPAC receptors. Our studies demonstrated that application of 1 nM VIP enhanced NMDA currents by stimulating the VPAC receptors as the effect was blocked by VPAC receptor antagonist [Ac-Tyr(1), D-Phe(2)]GRF (1-29). This enhancement of NMDA currents was blocked by both Rp-cAMPS and PKI(14-22) (they are highly specific PKA inhibitors), but not by the specific PKC inhibitor, bisindolylmaleimide I. In addition, the VIP-induced enhancement of NMDA currents was accentuated by inhibition of phosphodiesterase 4, which inhibits the degradation of cAMP. This regulation of NMDA receptors also required the scaffolding protein AKAP. In contrast, the potentiation induced by high concentration of VIP (e.g., 100 nM) was mediated by PAC1-R as well as by Src kinase. Overall, these results show that VIP can regulate NMDA receptors through different receptors and signaling pathways.

Vasoactive intestinal polypeptide entrains circadian rhythms in astrocytes

Many mammalian cell types show daily rhythms in gene expression driven by a circadian pacemaker. For example, cultured astrocytes display circadian rhythms in Period1 and Period2 expression. It is not known, however, how or which intercellular factors synchronize and sustain rhythmicity in astrocytes. Because astrocytes are highly sensitive to vasoactive intestinal polypeptide (VIP), a neuropeptide released by neurons and important for the coordination of daily cycling, the authors hypothesized that VIP entrains circadian rhythms in astrocytes. They used astrocyte cultures derived from knock-in mice containing a bioluminescent reporter of PERIOD2 (PER2) protein, to assess the effects of VIP on the rhythmic properties of astrocytes. VIP induced a dose-dependent increase in the peak-to-trough amplitude of the ensemble rhythms of PER2 expression with maximal effects near 100 nM VIP and threshold values between 0.1 and 1 nM. VIP also induced dose- and phase-dependent shifts in PER2 rhythms and daily VIP administration entrained bioluminescence rhythms of astrocytes to a predicted phase angle. This is the first demonstration that a neuropeptide can entrain glial cells to a phase predicted by a phase-response curve. The authors conclude that VIP potently entrains astrocytes in vitro and is a candidate for coordinating daily rhythms among glia in the brain.

Distinct expression and regulation of the glutamate transporter isoforms GLT-1a and GLT-1b in cultured astrocytes from a rat model of amyotrophic lateral sclerosis (hSOD1G93A)

Impaired glutamate uptake associated with accumulation of extracellular glutamate is a well-documented feature of amyotrophic lateral sclerosis (ALS) and related excitotoxicity is frequently proposed to participate in the progression of the disease. We herein characterised the expression and activity of the glutamate transporter glutamate transporter 1 (GLT-1) in cultured cortical astrocytes derived from a transgenic rat strain expressing an ALS-related mutated form of human superoxide dismutase 1 (hSOD1(G93A)). Measurements of d-[(3)H]-aspartate uptake velocity in the presence of selective glutamate transporter blockers demonstrated that astrocytes from the transgenic rats showed an impaired GLT-1 activity as compared to cells from wild-type animals. In addition, the density of GLT-1a mRNA in cells from hSOD1(G93A) animals appeared nearly 2-fold lower while the density of GLT-1b mRNA was nearly 2-fold higher. Besides, we observed that exposing the astrocytes from hSOD1(G93A) rats to the neuroprotective transmitter Peptide Histidine Isoleucine (PHI) for 24h caused a 4.5-fold increase in the GLT-1b mRNA level without influencing the expression of the other key isoform GLT-1a. This selective upregulation of GLT-1b by the neuropeptide was correlated with a significant increase in d-[(3)H]-aspartate uptake activity. The possibility to specifically regulate a single isoform of the high-affinity transporter GLT-1 is an unprecedented observation which sheds light on new perspectives for the pharmacological manipulation of glutamate transmission in diseases such as ALS.

Cultured astrocytes derived from corpus callosum or cortical grey matter show distinct glutamate handling properties

While the astrocytic control of extracellular glutamate concentration at synaptic contacts is well characterized, little is known regarding the clearance of glutamate along axon tracts, even though local excitotoxic damage has been reported. Therefore, we have compared glutamate handling in astrocyte cultures derived from white matter (corpus callosum) and grey matter tissues (cortical structures). These populations of astrocytes showed clearly distinct phenotypes, adopting stellate or protoplasmic morphologies respectively. In addition, white matter astrocytes showed high densities of the intermediate filament proteins glial fibrillary acidic protein, vimentin and nestin. The glutamate-aspartate transporter and glutamate transporter-1, as well as glutamine synthetase, were found to be expressed at higher levels in white matter compared with grey matter astrocytes. Consistent with this aspartate uptake capacity was three to fourfold higher in white matter cells, and the use of specific inhibitors revealed a substantial activity of glutamate transporter-1, contrasting with grey matter cells where this transporter appeared poorly functional. In addition, expression of type 5 metabotropic glutamate receptors was considerably higher in white matter astrocytes where the agonist (S)-3,5-dihydroxyphenylglycine triggered a large release of intracellular calcium. Differences in these astrocyte cultures were also observed when exposed to experimental conditions that trigger glial activation. This study highlights typical features of cultured astrocytes derived from white matter tissues, which appear constitutively adapted to handle excitotoxic insults. Moreover, the expression and activity of the astroglial components involved in the control of glutamatergic transmission are reinforced when these cells are maintained under conditions mimicking a gliotic environment.

Suppressive action of pituitary adenylate cyclase activating polypeptide (PACAP) on proliferation of immature mouse Leydig cell line TM3 cells

We aimed in the current study to understand the participation of PACAP in stage-specific Leydig and Sertoli cell functions. For this purpose, clonal cell lines TM3 (Leydig) and TM4 (Sertoli) cells, derived from the testis of immature BALB/c mice, were used. PACAP-specific receptors were detected in TM3 cells, but not in TM4 cells, which were characterized as PAC1 (type I) receptors. Stimulation of cAMP accumulation and testosterone secretion were observed in TM3 cells during 1-2 h treatment with PACAP38 (10(-10)-10(-7) M) or PACAP27 (10(-11)-10(-7) M). After around 10 h treatment with 10(-11)-10(-7) M PACAP38 or PACAP27, proliferation of TM3 cells was suppressed in time- and dose-dependent manners, which was confirmed by real-time cell electronic sensing (RT-CES) system and phase-contrast microscopy. At 6 h after the addition of PACAP38, the percent cell population in G(2)/M phases increased significantly, while that in S phase showed significant decrease with little change in G(0)/G(1) phases. The results revealed that PACAP exerts, in addition to early stimulatory effect on cAMP formation-steroidogenesis, sustained suppressive effect on cell proliferation in TM3 cells by controlling progression of the cell cycle. The suppressive action of PACAP on proliferation in TM3 cells supports the stage-specific participation of the peptide in differentiation of immature mouse Leydig cells.