Protein kinase C-dependent coupling of alpha(2A/D)-adrenergic receptors to phospholipase D

Fates of neurotrophins after retrograde axonal transport: phosphorylation of p75NTR is a sorting signal for delayed degradation

Neurotrophins can mediate survival or death of neurons. Opposing functions of neurotrophins are based on binding of these ligands to two distinct types of receptors: trk receptors and p75NTR. Previous work showed that target-derived NGF induces cell death, whereas BDNF and NT-3 enhance survival of neurons in the isthmo-optic nucleus of avian embryos. To determine the fate of retrogradely transported neurotrophins and test whether their sorting differs between neurotrophins mediating survival- or death-signaling pathways, we traced receptor-binding, sorting, and degradation kinetics of target-applied radiolabeled neurotrophins that bind in this system to trk receptors (BDNF, NT-3) or only to p75NTR (NGF). At the ultrastructural level, the p75NTR-bound NGF accumulates with a significant delay in multivesicular bodies and organelles of the degradation pathway on arrival in the cell body when compared with trk-bound BDNF or NT-3. This delayed lysosomal accumulation was restricted to target-derived NGF, but was not seen when NGF was supplied to the soma in vitro. The kinase inhibitors K252a and Gö6976 alter the kinetics of organelle accumulation: phosphorylation of p75NTR is a sorting signal for delayed sequestering of p75NTR-bound NGF in multivesicular bodies and delayed degradation in lysosomes when compared with trk-bound neurotrophins. Mutagenesis and mass spectrometry studies indicate that p75NTR is phosphorylated by conventional protein kinase C on serine 266. We conclude that, in addition to the known phosphorylation of trks, the phosphorylation of p75NTR can also significantly affect neuronal survival in vivo by changing the intracellular sorting and degradation kinetics of its ligands and thus signaling duration.

Resistance of macrophages to Mycobacterium avium is induced by alpha2-adrenergic stimulation

The ability of macrophages to control the growth of microorganisms is increased by macrophage activation. Previously, it was shown that epinephrine activated mouse macrophages to resist the growth of Mycobacterium avium via alpha(2)-adrenergic stimulation. In the present study, we show that the alpha(2)-adrenergic agonist (alpha(2)-agonist) clonidine induced resistance to M. avium growth in the RAW264.7 mouse macrophage cell line. The ability of catecholamines to induce resistance to mycobacteria was specific to alpha(2)-adrenergic stimulation, as alpha(1)-, beta(1)-, and beta(2)-agonists had no effect. Receptor signaling through Gi proteins was required. A G-protein antagonist specific for the alpha subunits of the Go/Gi family blocked the increased resistance induced by clonidine, while a Gs-protein antagonist was without effect. Both nitric oxide (NO) production and superoxide (O(2)(-)) production were required for the increased resistance to M. avium growth induced by clonidine. Although NO production was required, clonidine did not increase the level of NO in M. avium-infected cells. Since NO and O(2)(-) interact to produce peroxynitrite (ONOO(-)), we examined whether ONOO(-) mediates the increased resistance to M. avium induced by clonidine. 5,10,15,20-Tetrakis(4-sulfonatophenyl)prophyrinato iron (III) chloride (FeTPPS), a specific scavenger of ONOO(-), inhibited the effect of clonidine on M. avium growth. Clonidine also increased the production of ONOO(-) in M. avium-infected RAW264.7 cells, as measured by the oxidation of 123-dihydrorhodamine and the production of nitrated tyrosine residues. We therefore conclude that alpha(2)-adrenergic stimulation activates macrophages to resist the growth of M. avium by enhancing the production of ONOO(-).