Evidence for the existence of cAMP-dependent protein kinase phosphorylation system associated with specific phosphoproteins in stable microtubules from rat cerebral cortex

Chronic antidepressant treatment prevents accumulation of gsalpha in cholesterol-rich, cytoskeletal-associated, plasma membrane domains (lipid rafts)

Previous studies demonstrated that Gsalpha migrates from a Triton X-100 (TTX-100) insoluble membrane domain to a TTX-100 soluble membrane domain in response to chronic treatment with the antidepressants desipramine and fluoxetine. Antidepressant treatment also causes a Gsalpha redistribution in cells as seen by confocal microscopy. The current studies have focused on examining the possibility that the association between Gsalpha and the plasma membrane and/or cytoskeleton is altered in response to antidepressant treatment, and that this is relevant to both Gsalpha redistribution and the increased coupling between Gsalpha and adenylyl cyclase seen after chronic antidepressant treatment. Chronic treatment of C6 cells with two fuctionally and structurally distinct antidepressants, desipramine and fluoxetine, decreased the Gsalpha content of TTX-100 insoluble membrane domains by as much as 60%, while the inactive fluoxetine analog LY368514 had no effect. Disruption of these membrane domains with the cholesterol chelator methyl-beta-cyclodextrin altered the localization of many proteins involved in the cAMP signaling cascade, but only Gsalpha localization was altered by antidepressant treatment. In addition, microtubule disruption with colchicine elicited the movement of Gsalpha out of detergent-resistant membrane domains in a manner identical to that seen with antidepressant treatment. The data presented here further substantiate the role of Gsalpha as a major player in antidepressant-induced modification of neuronal signaling and also raise the possibility that an interaction between Gsalpha and the cytoskeleton is involved in this process.

Effect of reboxetine treatment on brain cAMP- and calcium/calmodulin-dependent protein kinases

Previous studies showed that the type II Ca(2+)/calmodulin- and cAMP-dependent protein kinases (CaMKII and PKA) are affected by long-term antidepressant treatment in presynaptic and somatodendritic compartments, respectively. This study describes the long-term effects of the selective noradrenaline reuptake inhibitor reboxetine on PKA and CaMKII, in both the microtubule and subsynaptosomal fractions of rat brain. Unlike other antidepressants, chronic reboxetine induced in the cerebrocortical soluble and microtubule fractions a decrease in the [(32)P]cAMP binding to the type II PKA regulatory subunit. No change in the cAMP-dependent endogenous phosphorylation of the protein substrate, microtubule-associated protein 2 was observed. In the hippocampal subsynaptosomal fractions (synaptic vesicles and synaptosomal membranes) reboxetine induced a robust increase in the activity but not in the expression of CaMKII. An increase in the calcium/calmodulin-dependent phosphorylation of presynaptic substrates was also detected. These findings showed that reboxetine modulates post-receptor signal transduction systems in rat brain.

Development of polarity in human erythroleukemia cells: roles of membrane ruffling and the centrosome

Cultured human erythroleukemia (HEL) cells were used to study the genesis of polarity in single cells. HEL cells grow in suspension in culture medium, but attach and spread on fibronectin when treated with 10 nM phorbol myristate acetate. If the spread cells are treated with dibutyryl cyclic adenosine monophosphate, about 50% of the cells polarize and form very striking elongated processes. Time-lapse video microscopy showed that elongation develops in these cells because the anterior pole of the cell, which bears a small ruffled membrane, moves slowly (approximately 0.16 microgram/min) forward on the substratum elongating the posterior pole or tail behind it. Using indirect immunofluorescence we found that elongation of the tail correlates with the development of long microtubule bundles emanating from the centrosome, which is located posterior to the nucleus on the trailing side of the cell. Incubation with nocodazole, which inhibited development of the long microtubules and the elongation, resulted in a centrosome positioned over the nucleus in 45% of the cells and extension of the membrane ruffling to many points around the cell's periphery. Unexpectedly, time-lapse video microscopy demonstrated that the treated cultures also contained some smaller cells with very marked anterior ruffles and short tails. These cells moved rapidly about the culture dish (maximum 0.8 microgram/min; average 0.5 microgram/min). In these fast moving cells the centrosome was also located posterior to the nucleus. Several recent reports have stressed the importance of relocation of the centrosome to an anterior position in cells developing polarity after experimental wounding. Our results show that both striking polarization and rapid motility can occur without such a relocation. The polarity induced in the HEL cells correlates most clearly with the limitation of membrane ruffling to one region; this limitation is removed by microtubule disassembly. We therefore propose that localized ruffling is the critical first step in polarized motility generally, and that centrosomal position is related to other factors.

Abnormalities of cAMP signaling in affective disorders: implication for pathophysiology and treatment

OBJECTIVE

During the last decade, much attention has been given to the role of signal transduction pathways in affective disorders. This review describes the possible role of the cAMP signaling in such disorders.

METHODS

Among the components of cAMP signaling, this review focuses on the cAMP-dependent phosphorylation system. We analyzed the basic components of the cAMP-dependent phosphorylation system and the preclinical evidence supporting their involvement in the biochemical action of antidepressants and mood stabilizers. The clinical data available until now, concerning the possible link between the cAMP-dependent phosphorylation system and the pathophysiology of affective disorders, are also reviewed.

RESULTS

The studies herein presented demonstrated that the levels and the activity of cAMP-dependent protein kinase are altered by antidepressants and mood stabilizers. Furthermore. these medications are able to modify the phosphorylation state, as well as the levels of some of the cAMP-dependent protein kinase substrates. More recently, clinical studies have reported abnormalities in the cAMP-dependent phosphorylation system in both peripheral cells and the postmortem brain of patients with affective disorders.

CONCLUSIONS

Overall, these studies support an involvement of cAMP signaling in affective disorders. The precise knowledge of the findings has the potential to improve the understanding of pharmacotherapy and to provide directions for the development of novel biochemical and genetic research strategies on the pathogenesis of affective disorders.

Second messenger-regulated protein kinases in the brain: their functional role and the action of antidepressant drugs

Depression has been treated pharmacologically for over three decades, but the views regarding the mechanism of action of antidepressant drugs have registered recently a major change. It was increasingly appreciated that adaptive changes in postreceptor signaling pathways, rather than primary action of drugs on monoamine transporters, metabolic enzymes, and receptors, are connected to therapeutic effect. For some of the various signaling pathways affected by antidepressant treatment, it was shown that protein phosphorylation, which represents an obligate step for most pathways, is markedly affected by long-term treatment. Changes were reported to be induced in the function of protein kinase C, cyclic AMP-dependent protein kinase, and calcium/calmodulin-dependent protein kinase. For two of these kinases (cyclic AMP- and calcium/calmodulin-dependent), the changes have been studied in isolated neuronal compartments (microtubules and presynaptic terminals). Antidepressant treatment activates the two kinases and increases the endogenous phosphorylation of selected substrates (microtubule-associated protein 2 and synaptotagmin). These modifications may be partly responsible for the changes induced by antidepressants in neurotransmission. The changes in protein phosphorylation induced by long-term antidepressant treatment may contribute to explain the therapeutic action of antidepressants and suggest new strategies of pharmacological intervention.

cAMP-dependent phosphorylation system after short and long-term administration of moclobemide

Accumulating evidence suggested that signal transduction cascade including protein phosphorylation is implicated in the neurochemical action of antidepressant agents. Clinical data indicated that moclobemide, a short acting and reversible inhibitor of monoamino oxidase type. A, is an effective antidepressant medication. However, little is known about the intracellular effects of this compound. Thus, in the present study we assessed the binding of cAMP to cAMP-dependent protein kinase (PKA) in rat cerebral cortex following short and long-term administration of moclobemide. The results showed that 21 days of treatment with moclobemide significantly increased the specific [32P]-cAMP covalent binding into the soluble 52-54 kDa cAMP-receptor. This effect was not seen following 1, 5 and 12 days of treatment. These findings suggest that PKA could be implicated in the biochemical effects of moclobemide.

Inhibitory effect of lithium on cAMP dependent phosphorylation system

The aim of the present study was to assess the direct effect of lithium on cAMP dependent phosphorylation. The results show that lithium, but not rubidium, at therapeutic and high concentrations significantly decreases the cAMP stimulated MAP2 endogenous phosphorylation in microtubule fraction. An inhibitory effect of lithium has also been found using purified heat stable microtubule proteins phosphorylated by the catalytic subunit of PKA. These data suggest a direct effect of lithium on the cAMP dependent protein kinase.

Association of mitogen-activated protein kinases with microtubules in mouse macrophages

Taxol, a microtubule-binding diterpene, mimics many effects of lipopolysaccharide (LPS) on mouse macrophages. The LPS-mimetic effects of taxol appear to be under the same genetic control as responses to LPS itself. Thus we have postulated a role for microtubule-associated proteins (MAP) in the response of macrophages to LPS. Stimulation of macrophages by LPS quickly induces the activation of mitogen-activated protein kinases (MAPK). MAPK are generally considered cytosolic enzymes. Herein we report that much of the LPS-activatable pool of MAPK in primary mouse peritoneal macrophages is microtubule associated. By immunofluorescence, MAPK were localized to colchicine- and nocodazole-disruptible filaments. From both mouse brain and RAW 264.7 macrophages, MAPK could be coisolated with polymerized tubulin. Fractionation of primary macrophages into cytosol-, microfilament-, microtubule-, and intermediated filament-rich extracts revealed that approximately 10% of MAPK but none of MAPK kinase (MEK1A and MEK2) was microtubule bound. Exposure of macrophages to LPS did not change the proportion of MAPK bound to microtubules, but preferentially activated the microtubule-associated pool. These findings confirm the prediction that LPS activates a kinase bound to microtubules. Together with LPS-mimetic actions of taxol and the shared genetic control of responses to LPS and taxol, these results support the hypothesis that a major LPS-signaling pathway in mouse macrophages may involve activation of one or more microtubule-associated kinases.

Effects of fluvoxamine on the protein phosphorylation system associated with rat neuronal microtubules

We have studied the phosphorylation system associated with the rat cerebrocortical microtubule fraction after short- and long-term administration (15 mg/kg) of fluvoxamine, a selective serotonin reuptake inhibitor with antidepressant activity. Fluvoxamine administered for 5 days significantly enhanced the 32P incorporation stimulated by cAMP into MAP2, while it failed to produce this effect after 12 and 21 days. Moreover, in the same periods of treatment no changes were observed in basal phosphorylation and in the pattern of microtubule proteins. In conclusion, our results suggest that changes in the protein phosphorylation system associated with the microtubule fraction could represent an early neurochemical modification involved in the action of fluvoxamine.