Expression of somatostatin and somatostatin receptor subtypes in Apolipoprotein D (ApoD) knockout mouse brain: An immunohistochemical analysis

The Lipocalin Apolipoprotein D Functional Portrait: A Systematic Review

Apolipoprotein D is a chordate gene early originated in the Lipocalin protein family. Among other features, regulation of its expression in a wide variety of disease conditions in humans, as apparently unrelated as neurodegeneration or breast cancer, have called for attention on this gene. Also, its presence in different tissues, from blood to brain, and different subcellular locations, from HDL lipoparticles to the interior of lysosomes or the surface of extracellular vesicles, poses an interesting challenge in deciphering its physiological function: Is ApoD a moonlighting protein, serving different roles in different cellular compartments, tissues, or organisms? Or does it have a unique biochemical mechanism of action that accounts for such apparently diverse roles in different physiological situations? To answer these questions, we have performed a systematic review of all primary publications where ApoD properties have been investigated in chordates. We conclude that ApoD ligand binding in the Lipocalin pocket, combined with an antioxidant activity performed at the rim of the pocket are properties sufficient to explain ApoD association with different lipid-based structures, where its physiological function is better described as lipid-management than by long-range lipid-transport. Controlling the redox state of these lipid structures in particular subcellular locations or extracellular structures, ApoD is able to modulate an enormous array of apparently diverse processes in the organism, both in health and disease. The new picture emerging from these data should help to put the physiological role of ApoD in new contexts and to inspire well-focused future research.

Multi-Transcript Level Profiling Revealed Distinct mRNA, miRNA, and tRNA-Derived Fragment Bio-Signatures for Coping Behavior Linked Haplotypes in HPA Axis and Limbic System

The molecular basis of porcine coping behavior (CB) relies on a sophisticated interplay of genetic and epigenetic features. Deep sequencing technologies allowed the identification of a plethora of new regulatory small non-coding RNA (sncRNA). We characterized mRNA and sncRNA profiles of central parts of the physiological stress response system including amygdala, hippocampus, hypothalamus and adrenal gland using systems biology for integration. Therefore, ten each of high- (HR) and low- (LR) reactive pigs (n = 20) carrying a CB associated haplotype in a prominent QTL-region on SSC12 were selected for mRNA and sncRNA expression profiling. The molecular markers related to the LR group included ATP1B2, MPDU1, miR-19b-5p, let-7g-5p, and 5′-tiRNA^Leu in the adrenal gland, miR-194a-5p, miR-125a-5p, miR-7-1-5p, and miR-107-5p in the hippocampus and CBL and PVRL1 in the hypothalamus. Interestingly, amygdalae of the LR group showed 5′-tiRNA and 5′-tRF (5′-tRF^Lys, 5′-tiRNA^Lys, 5′-tiRNA^Cys, and 5′-tiRNA^Gln) enrichment. Contrarily, molecular markers associated with the HR group encompassed miR-26b-5p, tRNA^Arg, tRNA^GlyiF in the adrenal gland, IGF1 and APOD in the amygdala and PBX1, TOB1, and C18orf1 in the hippocampus and miR-24 in the hypothalamus. In addition, hypothalami of the HR group were characterized by 3′-tiRNA enrichment (3′-tiRNAGln, 3′-tiRNA^Asn, 3′-tiRNA^Val, 3′-tRF^Pro, 3′-tiRNA^Cys, and 3′-tiRNA^Ala) and 3′-tRFs enrichment (3′-tRF^Asn, 3′-tRF^Glu, and 3′-tRF^Val). These evidence suggest that tRNA-derived fragments and their cleavage activity are a specific marker for coping behavior. Data integration revealed new bio-signatures of important molecular interactions on a multi-transcript level in HPA axis and limbic system of pigs carrying a CB-associated haplotype.

Role of Somatostatin in the Regulation of Central and Peripheral Factors of Satiety and Obesity

Obesity is one of the major social and health problems globally and often associated with various other pathological conditions. In addition to unregulated eating behaviour, circulating peptide-mediated hormonal secretion and signaling pathways play a critical role in food intake induced obesity. Amongst the many peptides involved in the regulation of food-seeking behaviour, somatostatin (SST) is the one which plays a determinant role in the complex process of appetite. SST is involved in the regulation of release and secretion of other peptides, neuronal integrity, and hormonal regulation. Based on past and recent studies, SST might serve as a bridge between central and peripheral tissues with a significant impact on obesity-associated with food intake behaviour and energy expenditure. Here, we present a comprehensive review describing the role of SST in the modulation of multiple central and peripheral signaling molecules. In addition, we highlight recent progress and contribution of SST and its receptors in food-seeking behaviour, obesity (orexigenic), and satiety (anorexigenic) associated pathways and mechanism.

δ-opioid receptor and somatostatin receptor-4 heterodimerization: possible implications in modulation of pain associated signaling

Pain relief is the principal action of opioids. Somatostatin (SST), a growth hormone inhibitory peptide is also known to alleviate pain even in cases when opioids fail. Recent studies have shown that mice are prone to sustained pain and devoid of analgesic effect in the absence of somatostatin receptor 4 (SSTR4). In the present study, using brain slices, cultured neurons and HEK-293 cells, we showed that SSTR4 and δ-Opioid receptor (δOR) exist in a heteromeric complex and function in synergistic manner. SSTR4 and δOR co-expressed in cortical/striatal brain regions and spinal cord. Using cultured neuronal cells, we describe the heterogeneous complex formation of SSTR4 and δOR at neuronal cell body and processes. Cotransfected cells display inhibition of cAMP/PKA and co-activation of SSTR4 and δOR oppose receptor trafficking induced by individual receptor activation. Furthermore, downstream signaling pathways either associated with withdrawal or pain relief are modulated synergistically with a predominant role of SSTR4. Inhibition of cAMP/PKA and activation of ERK1/2 are the possible cellular adaptations to prevent withdrawal induced by chronic morphine use. Our results reveal direct intra-membrane interaction between SSTR4 and δOR and provide insights for the molecular mechanism for the anti-nociceptive property of SST in combination with opioids as a potential therapeutic approach to avoid undesirable withdrawal symptoms.

Sleep-deprivation induces changes in GABA(B) and mGlu receptor expression and has consequences for synaptic long-term depression

Long term depression (LTD) in the CA1 region of the hippocampus, induced with a 20-Hz, 30 s tetanus to Schaffer collaterals, is enhanced in sleep-deprived (SD) rats. In the present study, we investigated the role of metabotropic glutamate receptors (mGluRs), γ-aminobutyric acid (GABA) B receptors (GABA(B)-Rs) and N-methyl-D-aspartic acid receptors (NMDARs) in the LTD of the population excitatory postsynaptic potential (pEPSP). The requirement of Ca(2+) from L- and T-type voltage-gated calcium channels (VGCCs) and intracellular stores was also studied. Results indicate that mGluRs, a release of Ca(2+) from intracellular stores and GABA(B)-Rs are required for LTD. Interestingly, while mGlu1Rs seem to be involved in both short-term depression and LTD, mGlu5Rs appear to participate mostly in LTD. CGP 55845, a GABA(B)-R antagonist, partially suppressed LTD in normally sleeping (NS) rats, while completely blocking LTD in SD rats. Moreover, GS-39783, a positive allosteric modulator for GABA(B)-R, suppressed the pEPSP in SD, but not NS rats. Since both mGluRs and GABA(B)-Rs seem to be involved in the LTD, especially in SD rats, we examined if the receptor expression pattern and/or dimerization changed, using immunohistochemical, co-localization and co-immunoprecipitation techniques. Sleep-deprivation induced an increase in the expression of GABA(B)-R1 and mGlu1αR in the CA1 region of the hippocampus. In addition, co-localization and heterodimerization between mGlu1αR/GABA(B)-R1 and mGlu1αR/GABA(B)-R2 is enhanced in SD rats. Taken together, our findings present a novel form of LTD sensitive to the activation of mGluRs and GABA(B)-Rs, and reveal, for the first time, that sleep-deprivation induces alterations in the expression and dimerization of these receptors.

Somatostatin receptor 1 and 5 double knockout mice mimic neurochemical changes of Huntington's disease transgenic mice

BACKGROUND

Selective degeneration of medium spiny neurons and preservation of medium sized aspiny interneurons in striatum has been implicated in excitotoxicity and pathophysiology of Huntington's disease (HD). However, the molecular mechanism for the selective sparing of medium sized aspiny neurons and vulnerability of projection neurons is still elusive. The pathological characteristic of HD is an extensive reduction of the striatal mass, affecting caudate putamen. Somatostatin (SST) positive neurons are selectively spared in HD and Quinolinic acid/N-methyl-D-aspartic acid induced excitotoxicity, mimic the model of HD. SST plays neuroprotective role in excitotoxicity and the biological effects of SST are mediated by five somatostatin receptor subtypes (SSTR1-5).

METHODS AND FINDINGS

To delineate subtype selective biological responses we have here investigated changes in SSTR1 and 5 double knockout mice brain and compared with HD transgenic mouse model (R6/2). Our study revealed significant loss of dopamine and cAMP regulated phosphoprotein of 32 kDa (DARPP-32) and comparable changes in SST, N-methyl-D-aspartic acid receptors subtypes, calbindin and brain nitric oxide synthase expression as well as in key signaling proteins including calpain, phospho-extracellular-signal-regulated kinases1/2, synapsin-IIa, protein kinase C-α and calcineurin in SSTR1/5(-/-) and R6/2 mice. Conversely, the expression of somatostatin receptor subtypes, enkephalin and phosphatidylinositol 3-kinases were strain specific. SSTR1/5 appears to be important in regulating NMDARs, DARPP-32 and signaling molecules in similar fashion as seen in HD transgenic mice.

CONCLUSIONS

This is the first comprehensive description of disease related changes upon ablation of G- protein coupled receptor gene. Our results indicate that SST and SSTRs might play an important role in regulation of neurodegeneration and targeting this pathway can provide a novel insight in understanding the pathophysiology of Huntington's disease.

Decreased kainate receptors in the hippocampus of apolipoprotein D knockout mice

Apolipoprotein D (ApoD) has many actions critical to maintaining mammalian CNS function. It is therefore significant that levels of ApoD have been shown to be altered in the CNS of subjects with schizophrenia, suggesting a role for ApoD in the pathophysiology of the disorder. There is also a large body of evidence that cortical and hippocampal glutamatergic, serotonergic and cholinergic systems are affected by the pathophysiology of schizophrenia. Thus, we decided to use in vitro radioligand binding and autoradiography to measure levels of ionotropic glutamate, some muscarinic and serotonin 2A receptors in the CNS of ApoD(-/-) and isogenic wild-type mice. These studies revealed a 20% decrease (mean+/-SEM: 104+/-10.2 vs. 130+/-10.4 fmol/mg ETE) in the density of kainate receptors in the CA 2-3 of the ApoD(-/-) mice. In addition there was a global decrease in AMPA receptors (F(1,214)=4.67, p<0.05) and a global increase in muscarinic M2/M4 receptors (F(1,208)=22.77, p<0.0001) in the ApoD(-/-) mice that did not reach significance in any single cytoarchitectural region. We conclude that glutamatergic pathways seem to be particularly affected in ApoD(-/-) mice and this may contribute to the changes in learning and memory, motor tasks and orientation-based tasks observed in these animals, all of which involve glutamatergic neurotransmission.

Functional effects of somatostatin receptor 1 activation on synaptic transmission in the mouse hippocampus

Somatostatin-14 (SRIF) co-localizes with GABA in the hippocampus and regulates neuronal excitability. A role of SRIF in the control of hippocampal activity has been proposed, although the exact contribution of each SRIF receptor (sst(1)-sst(5)) in mediating SRIF action requires some clarification. We used hippocampal slices of wild-type and sst(1) knockout (KO) mice and selective pharmacological tools to provide conclusive evidence for a role of sst(1) in mediating SRIF inhibition of synaptic transmission. With single- and double-label immunohistochemistry, we determined the distribution of sst(1) in hippocampal slices and we quantified sst(1) colocalization with SRIF. With electrophysiology, we found that sst(1) activation with CH-275 inhibited both the NMDA- and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-mediated responses. Results from sst(1) KO slices confirmed the specificity of CH-275 effects; sst(1) activation did not affect the inhibitory transmission which was in contrast increased by sst(4) activation with L-803,087 in both wild-type and sst(1) KO slices. The AMPA-mediated responses were increased by L-803,087. Functional interaction between sst(1) and sst(4) is suggested by the finding that their combined activation prevented the CH-275-induced inhibition of AMPA transmission. The involvement of pre-synaptic mechanisms in mediating inhibitory effects of sst(1) on excitatory transmission was demonstrated by the finding that CH-275 (i) increased the paired-pulse facilitation ratio, (ii) did not influence the AMPA depolarization in the presence of tetrodotoxin, and (iii) inhibited glutamate release induced by epileptiform treatment. We conclude that SRIF control of excitatory transmission through an action at sst(1) may represent an important contribution to the regulation of hippocampal activity.