Down-regulation of phospholipase C-beta1 following chronic muscarinic receptor activation

Phospholipase C beta 1 expression in the dorsolateral prefrontal cortex from patients with schizophrenia at different stages of illness

OBJECTIVE

Our recent microarray study detected decreases in the expression of phospholipase C beta 1 mRNA in the dorsolateral prefrontal cortex from subjects with schizophrenia at different stages of illness. Thus we aimed to validate and extend these findings.

METHOD

We measured levels of mRNA and protein for phospholipase C beta 1 variant a and b using real-time PCR and western blot analysis, respectively, in the dorsolateral prefrontal cortex from subjects with schizophrenia, who had a short (< 7 years) or long (> 22 years) duration of illness.

RESULTS

Compared to age/sex matched controls, levels of phospholipase C beta 1 variant a and b mRNAs were decreased (-33% and -50%, respectively) in short duration schizophrenia. By contrast, only variant a mRNA was decreased (-24%) in long duration schizophrenia. There was no significant difference in the protein levels of either phospholipase C beta 1 variant in schizophrenia, irrespective of duration of illness (variant a; P = 0.84, variant b; P = 0.73).

CONCLUSION

Our data confirm that phospholipase C beta 1 transcript levels are decreased in the dorsolateral prefrontal cortex from subjects with schizophrenia. However, the changes in levels of mRNA do not translate into a change at the level of protein. It is possible protein expression is regulated independently of mRNA and it remains to be determined whether there is a functional consequence of this change in mRNA relating to the pathophysiology of schizophrenia.

Temporal profiling of changes in phosphatidylinositol 4,5-bisphosphate, inositol 1,4,5-trisphosphate and diacylglycerol allows comprehensive analysis of phospholipase C-initiated signalling in single neurons

Phosphatidylinositol 4,5-bisphosphate (PIP₂) fulfils vital signalling roles in an array of cellular processes, yet until recently it has not been possible selectively to visualize real-time changes in PIP₂ levels within living cells. Green fluorescent protein (GFP)-labelled Tubby protein (GFP-Tubby) enriches to the plasma membrane at rest and translocates to the cytosol following activation of endogenous Gα_q/11-coupled muscarinic acetylcholine receptors in both SH-SY5Y human neuroblastoma cells and primary rat hippocampal neurons. GFP-Tubby translocation is independent of changes in cytosolic inositol 1,4,5-trisphosphate and instead reports dynamic changes in levels of plasma membrane PIP₂. In contrast, enhanced GFP (eGFP)-tagged pleckstrin homology domain of phospholipase C (PLCδ1) (eGFP-PH) translocation reports increases in cytosolic inositol 1,4,5-trisphosphate. Comparison of GFP-Tubby, eGFP-PH and the eGFP-tagged C1₂ domain of protein kinase C-γ [eGFP-C1(2); to detect diacylglycerol] allowed a selective and comprehensive analysis of PLC-initiated signalling in living cells. Manipulating intracellular Ca²⁺ concentrations in the nanomolar range established that GFP-Tubby responses to a muscarinic agonist were sensitive to intracellular Ca²⁺ up to 100–200 nM in SH-SY5Y cells, demonstrating the exquisite sensitivity of agonist-mediated PLC activity within the range of physiological resting Ca²⁺ concentrations. We have also exploited GFP-Tubby selectively to visualize, for the first time, real-time changes in PIP₂ in hippocampal neurons.

Effect of chronic gestational treatment with caffeine or theophylline on Group I metabotropic glutamate receptors in maternal and fetal brain

Pregnant rats were treated throughout the gestational period with either caffeine or theophylline, and its effect on the metabotropic glutamate receptor (mGluRs) signal transduction pathway was studied in both maternal and fetal brain. In maternal brain, radioligand binding assays showed that chronic treatment with methylxanthines caused a significant decrease in the total number of mGluRs. This decrease was accompanied by an increase in receptor affinity. Immunodetection showed that mGluR1a and phospholipase C beta1 (PLCbeta1) were significantly decreased in response to chronic methylxanthine treatment, whereas alphaG(q/11) was not affected. A loss was also detected of PLC stimulation mediated by (S)-3,5-dihydroxyphenylglycine (DHPG), a selective Group I mGluR agonist, suggesting desensitization of the mGluR/PLC pathway. In fetal brain, a loss in total mGluRs was observed in fetuses from mothers treated with caffeine or theophylline, without variation in receptor affinity. A decrease in mGluR1a, alphaG(q/11) and PLCbeta1 levels was also observed in response to treatment. However, changes detected in this immature tissue were not associated with variations in PLC activity. These results suggest that chronic caffeine or theophylline treatment down-regulates several mGluR/PLC transduction pathway components in both maternal and fetal brain, causing a loss of receptor responsiveness only in maternal brain.

Desensitization of angiotensin-stimulated inositol phosphate accumulation in human vascular smooth muscle cells

The effect of angiotensin II treatment on desensitization of phospholipase C (PLC)-mediated inositol phosphate accumulation has not been quantitated in human aortic vascular smooth muscle (HVSM) cells. We determined the angiotensin II pretreatment dose dependency and time course for desensitization of PLC activation in HVSM cells and the effect of protein kinase C (PKC) activators on angiotensin II-mediated inositol phosphate accumulation. Our results with PKC activators and direct G protein stimulators suggest that PKC activation may play a negative feedback role in desensitization of angiotensin II-activated signaling in HVSM cells by modifying the Gq transducer, PLC-beta effector, or related proteins in the signaling pathway. However, neither angiotensin II nor PKC activator affected basal phosphorylation levels of PLC-beta1 or PLC-beta3 in HVSM cells; PLC-beta isoenzymes were shown to be phosphorylated in unstimulated cells independent of PKC inhibition. We suggest that desensitization of G protein-stimulated inositol phosphate accumulation in HVSM differs from other cell types in which phosphorylation of PLC-beta isoenzymes accompanies desensitization.