Circumventing embryonic lethality with Lcmt1 deficiency: generation of hypomorphic Lcmt1 mice with reduced protein phosphatase 2A methyltransferase expression and defects in insulin signaling

Protein phosphatase 2A (PP2A), the major serine/threonine phosphatase in eukaryotic cells, is a heterotrimeric protein composed of structural, catalytic, and targeting subunits. PP2A assembly is governed by a variety of mechanisms, one of which is carboxyl-terminal methylation of the catalytic subunit by the leucine carboxyl methyltransferase LCMT1. PP2A is nearly stoichiometrically methylated in the cytosol, and although some PP2A targeting subunits bind independently of methylation, this modification is required for the binding of others. To examine the role of this methylation reaction in mammalian tissues, we generated a mouse harboring a gene-trap cassette within intron 1 of Lcmt1. Due to splicing around the insertion, Lcmt1 transcript and LCMT1 protein levels were reduced but not eliminated. LCMT1 activity and methylation of PP2A were reduced in a coordinate fashion, suggesting that LCMT1 is the only PP2A methyltransferase. These mice exhibited an insulin-resistance phenotype, indicating a role for this methyltransferase in signaling in insulin-sensitive tissues. Tissues from these animals will be vital for the in vivo identification of methylation-sensitive substrates of PP2A and how they respond to differing physiological conditions.

Wortmannin reduces insulin signaling and death in seizure-prone Pcmt1-/- mice

L-isoaspartyl (D-aspartyl) O-methyltransferase deficient mice (Pcmt1^−/−) accumulate isomerized aspartyl residues in intracellular proteins until their death due to seizures at approximately 45 days. Previous studies have shown that these mice have constitutively activated insulin signaling in their brains, and that these brains are 20–30% larger than those from age-matched wild-type animals. To determine whether insulin pathway activation and brain enlargement is responsible for the fatal seizures, we administered wortmannin, an inhibitor of the phosphoinositide 3-kinase that catalyzes an early step in the insulin pathway. Oral wortmannin reduced the average brain size in the Pcmt1^−/− animals to within 6% of the wild-type DMSO administered controls, and nearly doubled the lifespan of Pcmt1^−/− at 60% survival of the original population. Immunoblotting revealed significant decreases in phosphorylation of Akt, PDK1, and mTOR in Pcmt1^−/− mice and Akt and PDK1 in wild-type animals upon treatment with wortmannin. These data suggest activation of the insulin pathway and its resulting brain enlargement contributes to the early death of Pcmt1−/− mice, but is not solely responsible for the early death observed in these animals.

The relationship between glutamate release and cerebral blood flow after focal cerebral ischaemia in the cat: effect of pretreatment with enadoline (a kappa receptor agonist)

The effect of the kappa-opioid agonist enadoline (CI-977) upon the relationship between cerebral blood flow and glutamate release was simultaneously assessed (using microdialysis and hydrogen clearance techniques respectively) at the same anatomical locus in the cerebral cortex (suprasylvian gyrus) after permanent middle cerebral artery (MCA) occlusion in halothane-anaesthetised cats. During controlled graded ischaemia, pretreatment with enadoline (0.3 mg/kg i.v. followed by continuous infusion at 0.15 mg/kg/h), initiated 30 min prior to MCA occlusion, significantly attenuated the marked increases in extracellular glutamate, aspartate and GABA observed in the focal ischaemic penumbra. The present data are consistent with the hypothesis that the neuroprotective efficacy of enadoline in focal cerebral ischaemia is due to inhibition of glutamate release in the ischaemic penumbra.

Pharmacological modification of glutamate neurotoxicity in vivo

The ability of five agents (dizocilpine [MK-801], 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline [NBQX], enadoline [CI-977], L-nitroarginine methyl ester [L-NAME] and BW 1003c87) with well defined, distinct pharmacological profiles and with established anti-ischemic efficacy, to modify neuronal damage has been examined in a simple in vivo model of glutamate excitotoxicity. Cortical lesions were produced in physiologically-monitored halothane-anesthetised rats by reverse dialysis of glutamate. The volume of the lesion was quantified histologically by image analysis of approximately 20 sections taken at 200 microm intervals throughout the lesion. The AMPA and NMDA receptor antagonists (NBQX and MK-801) and the inhibitor of nitric oxide synthase (L-NAME) significantly reduced the lesion volume by a similar extent (by approximately 30% from vehicle). Two agents (the kappa opioid agonist, CI-977 and the sodium channel blocker, BW 1003c87) which putatively inhibit the release of endogenous glutamate presynaptically, had dissimilar effects on lesion size. CI-977 failed to alter the amount of damage produced by exogenous glutamate, whereas BW 1003c87 reduced the lesion size by approximately 50%. Using this model, the neuroprotective effects of anti-ischemic drugs can be explored in vivo, uncomplicated in contrast to experimental ischemia by reduced oxygen delivery, drug effects on tissue blood flow and compromised energy generation. In consequence, additional mechanistic insight into anti-ischemic drug action in vivo can be obtained.

Atrial natriuretic peptide effects in AtT-20 pituitary tumour cells

Whether atrial natriuretic peptide (ANP)-evoked inhibition of corticotrophin-releasing factor (CRF)-stimulated ACTH secretion was also manifest in ACTH secreting AtT-20 pituitary tumour cells was investigated. ANP stimulated increases in cGMP accumulation at concentrations of the peptide above 10(-8) M which indicates the presence of the ANP receptors on these cells. CRF stimulated a concentration-dependent increase in ACTH secretion from AtT-20 cells which was unaffected by ANP, 8-bromo-cGMP, or sodium nitroprusside (SNP). Calcium stimulated a concentration-dependent increase in ACTH secretion from electrically permeabilised cells which was unaffected by co-incubation with cGMP but potentiated by cAMP. These results reveal the presence of ANP receptors on AtT-20 cells but suggest that an incomplete expression of the stimulus-secretion coupling mechanisms for ANP, at some point after cGMP production, prevents the effects of natriuretic peptides upon ACTH secretion being manifest in these cells.