Regulation of guanine nucleotide exchange through phosphorylation of eukaryotic initiation factor eIF2alpha. Role of the alpha- and delta-subunits of eiF2b

The guanine nucleotide exchange activity of eIF2B plays a key regulatory role in the translation initiation phase of protein synthesis. The activity is markedly inhibited when the substrate, i. e. eIF2, is phosphorylated on Ser51 of its alpha-subunit. Genetic studies in yeast implicate the alpha-, beta-, and delta-subunits of eIF2B in mediating the inhibition by substrate phosphorylation. However, the mechanism involved in the inhibition has not been defined biochemically. In the present study, we have coexpressed the five subunits of rat eIF2B in Sf9 cells using the baculovirus system and have purified the recombinant holoprotein to >90% homogeneity. We have also expressed and purified a four-subunit eIF2B complex lacking the alpha-subunit. Both the five- and four-subunit forms of eIF2B exhibit similar rates of guanine nucleotide exchange activity using unphosphorylated eIF2 as substrate. The five-subunit form is inhibited by preincubation with phosphorylated eIF2 (eIF2(alphaP)) and exhibits little exchange activity when eIF2(alphaP) is used as substrate. In contrast, eIF2B lacking the alpha-subunit is insensitive to inhibition by eIF2(alphaP) and is able to exchange guanine nucleotide using eIF2(alphaP) as substrate at a faster rate compared with five-subunit eIF2B. Finally, a double point mutation in the delta-subunit of eIF2B has been identified that results in insensitivity to inhibition by eIF2(alphaP) and exhibits little exchange activity when eIF2(alphaP) is used as substrate. The results provide the first direct biochemical evidence that the alpha- and delta-subunits of eIF2B are involved in mediating the effect of substrate phosphorylation.     

Parametric image reconstruction using spectral analysis of PET projection data

Spectral analysis is a general modelling approach that enables calculation of parametric images from reconstructed tracer kinetic data independent of an assumed compartmental structure. We investigated the validity of applying spectral analysis directly to projection data motivated by the advantages that: (i) the number of reconstructions is reduced by an order of magnitude and (ii) iterative reconstruction becomes practical which may improve signal-to-noise ratio (SNR). A dynamic software phantom with typical 2-[11C]thymidine kinetics was used to compare projection-based and image-based methods and to assess bias-variance trade-offs using iterative expectation maximization (EM) reconstruction. We found that the two approaches are not exactly equivalent due to properties of the non-negative least-squares algorithm. However, the differences are small (< 5%) and mainly affect parameters related to early and late time points on the impulse response function (K1 and, to a lesser extent, VD). The optimal number of EM iteration was 15-30 with up to a two-fold improvement in SNR over filtered back projection. We conclude that projection-based spectral analysis with EM reconstruction yields accurate parametric images with high SNR and has potential application to a wide range of positron emission tomography ligands.     

Expression of the thrombin receptor in developing bone and associated tissues

Thrombin, a serine protease with a central role in thrombosis and hemostasis, is also a specific agonist for a variety of cellular responses in osteoblasts and stimulates bone resorption in organ culture. Cultured osteoblast-like cells express the proteolytically activated thrombin receptor, but the significance of this finding in vivo remains unknown. Immunohistochemistry was used to investigate the normal tissue distribution of the proteolytically activated thrombin receptor in developing rat bones and associated tissues. In hind limbs, the receptor was first observed on embryonic day 16 and became more abundant within the limb as gestation progressed. Thrombin receptor staining was detected on osteoblasts, macrophages, muscle cells, and endothelial cells, but not osteoclasts. Similarly, osteoblasts in developing calvariae stained positively for the thrombin receptor. The pattern of receptor expression by primary osteoblast cultures and freshly isolated macrophages and osteoclasts corresponded to that observed in vivo. The observed pattern of thrombin receptor expression in bone cells supports the hypothesis that cell-mediated thrombin-induced bone resorption is mediated by osteoblasts.     

Expression of IL-16 in allergen-induced late-phase nasal responses and relation to topical glucocorticosteroid treatment

Allergen-induced late nasal responses (LNRs) are associated with a cellular infiltrate in which CD4+ cells are prominent. These cells have been shown to be the major cellular source of Th2-type cytokines. Mechanisms responsible for the local accumulation of CD4+ cells in the nasal mucosa after allergen exposure are unclear. IL-16 is a potent chemoattractant for CD4+ cells in vitro and may play a significant role in recruiting CD4+ cells in LNRs. We investigated the expression of IL-16 messenger RNA and immunoreactivity in nasal biopsy specimens from 17 subjects with allergic rhinitis. A biopsy specimen of the nasal inferior turbinate was obtained before and 24 hours after local nasal provocation with grass pollen extract after 6 weeks of treatment with either topical fluticasone propionate (n = 9) or placebo (n = 8) nasal spray twice daily. IL-16 mRNA-positive cells and IL-16-immunoreactive cells were identified in both the epithelium and the subepithelial tissue at baseline. Within the placebo-treated group, the numbers of epithelial and subepithelial IL-16 mRNA-positive cells and IL-16-immunoreactive cells were significantly increased 24 hours after challenge compared with baseline (p < 0.001). Topical glucocorticoid therapy resulted in a decrease in allergen-induced epithelial immunoreactive cells and subepithelial IL-16 mRNA-positive cells. The numbers of CD4+ cells increased after antigen challenge compared with baseline (p < 0.05), and this increase was inhibited by glucocorticoid treatment. There were significant correlations between epithelial and subepithelial IL-16 immunoreactivity and CD4+ cell infiltration after antigen challenge. The upregulation of IL-16 expression in allergic nasal mucosa after antigen challenge may have critical implications in the accumulation of CD4+ cells in response to antigen exposure. Steroid-mediated inhibition of IL-16 may be partly responsible for the decrease in local CD4+ cells after topical glucocorticoid therapy.     

Management of acute extrapyramidal effects induced by antipsychotic drugs

The management of acute extrapyramidal effects (EPEs) induced by antipsychotic drugs is reviewed. EPEs associated with antipsychotics include acute dystonias, pseudoparkinsonism, and akathisia. Acute dystonias consist of abnormal muscle spasms and postures and usually occur three to five days after antipsychotic therapy begins or the dosage is increased. Acute dystonias should be treated with anticholinergic medications or benzodiazepines. Antipsychotic-induced pseudoparkinsonism has the same clinical appearance as idiopathic parkinsonism. Symptoms generally appear within the first three months. Pseudoparkinsonism is managed by lowering the anti-psychotic dosage or by adding an anticholinergic agent or a mantadine; switching to a low-potency agent or an atypical antipsychotic may also help. Akathisia is characterized by subjective feelings of restlessness and anxiety and objective signs of motor activity, such as inability to sit still. This EPE appears days to weeks after antipsychotic exposure begins and can be difficult to manage. If reduction of the antipsychotic dosage or a switch to a less potent antipsychotic is not practical or effective, an anticholinergic, beta-blocker, or benzodiazepine may be added. Lipophilic beta-blockers, especially propranolol and metoprolol, appear to be the most effective treatments. Anticholinergic agents are commonly given to prevent acute dystonias, especially in high-risk patients, but long-term prophylaxis is controversial. Atypical antipsychotics may have less potential to induce EPEs. Options in the management of antipsychotic-associated EPEs include using the lowest effective dosage of antipsychotic, treating the reactions with medications, and changing the antipsychotic to one with less potential for inducing EPEs.     

Immortalization of osteoclast precursors by targeting Bcl -XL and Simian virus 40 large T antigen to the osteoclast lineage in transgenic mice

Cellular and molecular characterization of osteoclasts (OCL) has been extremely difficult since OCL are rare cells, and are difficult to isolate in large numbers. We used the tartrate-resistant acid phosphatase promoter to target the bcl-XL and/or Simian Virus 40 large T antigen (Tag) genes to cells in the OCL lineage in transgenic mice as a means of immortalizing OCL precursors. Immunocytochemical studies confirmed that we had targeted Bcl-XL and/or Tag to OCL, and transformed and mitotic OCL were readily apparent in bones from both Tag and bcl-XL/Tag mice. OCL formation in primary bone marrow cultures from bcl-XL, Tag, or bcl-XL/Tag mice was twofold greater compared with that of nontransgenic littermates. Bone marrow cells from bcl-XL/Tag mice, but not from singly transgenic bcl-XL or Tag mice, have survived in continuous culture for more than a year. These cells form high numbers of bone-resorbing OCL when cultured using standard conditions for inducing OCL formation, with approximately 50% of the mononuclear cells incorporated into OCL. The OCL that form express calcitonin receptors and contract in response to calcitonin. Studies examining the proliferative capacity and the resistance of OCL precursors from these transgenic mice to apoptosis demonstrated that the increased numbers of OCL precursors in marrow from bcl-XL/Tag mice was due to their increased survival rather than an increased proliferative capacity compared with Tag, bcl-XL, or normal mice. Histomorphometric studies of bones from bcl-XL/Tag mice also confirmed that there were increased numbers of OCL precursors (TRAP + mononuclear cells) present in vivo. These data demonstrate that by targeting both bcl-XL and Tag to cells in the OCL lineage, we have immortalized OCL precursors that form bone-resorbing OCL with an efficiency that is 300-500 times greater than that of normal marrow.