Ionising radiation induces the expression of PARP-1 and PARP-2 genes in Arabidopsis

By screening for Arabidopsis genes activated by ionising radiation (IR)-induced DNA damage, we have isolated a cDNA hybridising with a 3.2-kb mRNA that accumulates rapidly and strongly in irradiated cell suspensions or whole plants. The cDNA codes for a 110-kDa protein that is highly homologous to the 116-kDa vertebrate poly(ADP-ribose) polymerase (PARP-1). It is recognised by a human anti-PARP-1 antibody, binds efficiently to DNA strand interruptions in vitro, and catalyses DNA damage-dependent (ADP-ribose) polymer synthesis. We have named this protein AtPARP-1. We have also extended our observations to the Arabidopsis app (AtPARP-2) gene, demonstrating for the first time that IR-induced DNA strand interruptions induce rapid and massive accumulation of AtPARP-1 and AtPARP-2 transcripts, whereas dehydration and cadmium preferentially induce the accumulation of AtPARP-2 transcripts. The IR-induced PARP gene expression seen in Arabidopsis is in striking contrast to the post-translational activation of the PARP-1 protein that is associated with genotoxic stress in animal cells. AtPARP-1 transcripts accumulate in all plant organs after exposure to ionising radiation, but this is followed by an increase in AtPARP-1 protein levels only in tissues that contain large amounts of actively dividing cells. This cell-type specific accumulation of AtPARP-1 protein in response to DNA damage is compatible with a role for the AtPARP-1 protein in the maintenance of DNA integrity during replication, similar to the role of "guardian of the genome" attributed to its animal counterpart.

Lipid phosphate phosphatases in Arabidopsis. Regulation of the AtLPP1 gene in response to stress

An Arabidopsis thaliana gene (AtLPP1) was isolated on the basis that it was transiently induced by ionizing radiation. The putative AtLPP1 gene product showed homology to the yeast and mammalian lipid phosphate phosphatase enzymes and possessed a phosphatase signature sequence motif. Heterologous expression and biochemical characterization of the AtLPP1 gene in yeast showed that it encoded an enzyme (AtLpp1p) that exhibited both diacylglycerol pyrophosphate phosphatase and phosphatidate phosphatase activities. Kinetic analysis indicated that diacylglycerol pyrophosphate was the preferred substrate for AtLpp1p in vitro. A second Arabidopsis gene (AtLPP2) was identified based on sequence homology to AtLPP1 that was also heterologously expressed in yeast. The AtLpp2p enzyme also utilized diacylglycerol pyrophosphate and phosphatidate but with no preference for either substrate. The AtLpp1p and AtLpp2p enzymes showed differences in their apparent affinities for diacylglycerol pyrophosphate and phosphatidate as well as other enzymological properties. Northern blot analyses showed that the AtLPP1 gene was preferentially expressed in leaves and roots, whereas the AtLPP2 gene was expressed in all tissues examined. AtLPP1, but not AtLPP2, was regulated in response to various stress conditions. The AtLPP1 gene was transiently induced by genotoxic stress (gamma ray or UV-B) and elicitor treatments with mastoparan and harpin. The regulation of the AtLPP1 gene in response to stress was consistent with the hypothesis that its encoded lipid phosphate phosphatase enzyme may attenuate the signaling functions of phosphatidate and/or diacylglycerol pyrophosphate that form in response to stress in plants.

In vivo effect of methyl-quinuclidinyl-benzylate on myocardial beta-adrenoceptor density

The muscarinic receptor antagonist methyl-quinuclidinyl-benzylate decreased myocardial beta-adrenoceptor density Bmax: 20.4 +/- 2.4 pmol/ml tissue versus 33.3 +/- 4 pmol/ml tissue in control dogs (P < 0.001), as assessed by using [11C]CGP-12177 (((2S)-4-(3-t-butyl-amino-2 hydroxypropoxy)-benzimidazol-2-one)) and positron emission tomography. In contrast, atropine did not induce any change in Bmax: 33.7 +/- 3.6 pmol/ml tissue. We hypothetized that methyl-quinuclidinyl-benzylate induced the release of norepinephrine from sympathetic nerve terminals, an effect which could be blocked by guanethidine. Guanethidine alone (10 mg/kg) did not change Bmax: 35.5 +/- 6 pmol/ml tissue. Guanethidine + methyl-quinuclidinyl-benzylate did not induce any significant change in Bmax: 31.5 +/- 5.1 pmol/ml tissue. Therefore, it seems likely that methyl-quinuclidinyl-benzylate acts at the presynaptic level, probably inducing the release of norepinephrine which then causes a down-regulation of beta-adrenoceptors.

Preparation and pharmacological characterization of [76Br]-meta-bromobenzylguanidine ([76Br]MBBG)

[76Br]-meta-Bromobenzylguanidine ([76Br]MBBG) was prepared from the iodinated analog (MIBG) and [76Br]NH4 using a Cu(+)-assisted halogen exchange reaction. [76Br]MBBG was produced in a 60-65% radiochemical yield with a specific activity of 20 MBq/nmol. In rats, biodistribution kinetic studies showed a high uptake of [76Br]MBBG in heart tissues with its maximum of 5% ID/S at 2 h p.i.; whereas 4 h p.i., the maximum of the heart-to-lung concentration ratio of 8 was observed. Metabolic studies in rats indicated that [76Br]MBBG was rapidly metabolized in plasma. However in heart tissue, 25 h p.i., 85% of the radioactivity still represented unchanged radiotracer. Pharmacological studies in rats showed that the myocardial uptake of [76Br]MBBG was similar to that of norepinephrine. After pretreatment of the rats, the uptake of [76Br]MBBG was reduced 4 h p.i. to the following values: after desipramine (DMI) to 37%, after dexamethasone (DXM) to 88% and after 6-hydroxydopamine (6-OHDA) to 16%. These preliminary results suggest that [76Br]MBBG can be useful for the assessment of heart catecholamine reuptake disorders with PET.