Two synaptojanin 1 isoforms are recruited to clathrin-coated pits at different stages

Phosphoinositides are thought to play an important role in clathrin-coated pit (CCP) dynamics. Biochemical and structural studies have shown a direct interaction of phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P2] with endocytic clathrin adaptors, whereas functional studies using cell-free systems or intact cells have demonstrated the importance of PI(4,5)P2 synthesis and dephosphorylation in clathrin coating and uncoating, respectively. Furthermore, genetic manipulations of kinases and phosphatases involved in PI(4,5)P2 metabolism result in major defects in synaptic vesicle recycling and other forms of clathrin-dependent endocytosis. However, live imaging studies of these enzymes at CCPs have not been conducted. We have used multicolor total internal reflection fluorescence microscopy (TIRFM) to visualize the spatial-temporal recruitment of synaptojanin 1 (SJ1), a polyphosphoinositide phosphatase, and its binding partner endophilin to CCPs. Strikingly, we observed differential temporal recruitment of the two major SJ1 splice variants to CCPs. The 145-kDa isoform, the predominant isoform expressed in the brain, was rapidly recruited as a "burst," together with endophilin, at a late stage of CCP formation. In contrast, the nonneuronal ubiquitously expressed 170-kDa isoform of SJ1 was present at all stages of CCP formation. These results raise the possibility that dynamic phosphoinositide metabolism may occur throughout the lifetime of a CCP.     

Regulation of postsynaptic AMPA responses by synaptojanin 1

Endocytosis of postsynaptic AMPA receptors is a mechanism through which efficiency of neurotransmission is regulated. We have genetically tested the hypothesis that synaptojanin 1, a phosphoinositide phosphatase implicated in the endocytosis of synaptic vesicles presynaptically, may also function in the endocytosis of AMPA receptors postsynaptically. Electrophysiological recordings of cultured hippocampal neurons showed that miniature excitatory postsynaptic current amplitudes were larger in synaptojanin 1 knockout (KO) neurons because of an increase of surface-exposed AMPA receptors. This change did not represent an adaptive response to decreased presynaptic release in KO cultures and was rescued by the expression of wild type, but not catalytically inactive synaptojanin 1, in the postsynaptic neuron. NMDA-induced internalization of pHluorin-tagged AMPA receptors (GluR2) was impaired in KO neurons. These results reveal a function of synaptojanin 1 in constitutive and triggered internalization of AMPA receptors and thus indicate a role for phosphatidylinositol(4,5)-bisphosphate metabolism in the regulation of postsynaptic AMPA responses.     

Inositol hexakisphosphate promotes dynamin I- mediated endocytosis

Membrane homeostasis is maintained by exocytosis and endocytosis. The molecular mechanisms regulating the interplay between these two processes are not clear. Inositol hexakisphosphate (InsP(6)) is under metabolic control and serves as a signal in the pancreatic beta cell stimulus-secretion coupling by increasing Ca(2+)-channel activity and insulin exocytosis. We now show that InsP(6) also promotes dynamin I-mediated endocytosis in the pancreatic beta cell. This effect of InsP(6) depends on calcineurin-induced dephosphorylation and is accounted for by both activation of protein kinase C and inhibition of the phosphoinositide phosphatase synaptojanin and thereby formation of phosphatidylinositol 4,5-bisphosphate. In regulating both exocytosis and endocytosis, InsP(6) thus may have an essential integral role in membrane trafficking.     

Rho-mediated activation of PI(4)P5K and lipid second messengers is necessary for promotion of angiogenesis by Semaphorin 4D

Phosphatidylinositol 4-phosphate 5-kinase (PI(4)P5K) is a type I lipid kinase that generates the lipid second messenger phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and functions downstream of RhoA in actin organization. It is known to play an essential role in neurite remodeling, yielding a phenotype identical to that seen in cells treated with Semaphorin 4D (Sema4D), a protein that regulates proliferation, adhesion and migration in many different cell types. Plexin-B1, the receptor for Sema4D, activates RhoA in order to generate a pro-angiogenic signal in endothelial cells. Therefore, we looked in human umbilical vein endothelial cells (HUVEC) to determine if Plexin-B1 exerted control over the cytoskeleton by regulation of PI(4)P5K activity. Here we demonstrate the Rho/Rho Kinase (ROK)-dependent generation of PI(4,5)P(2) upon treatment of HUVEC with Sema4D, as well as co-localization of PI(4)P5Kα with Plexin-B1. Formation of PI(4,5)P(2) was necessary for cytoskeletal polymerization, as expression of the phosphatase synaptojanin blocked this effect. We noted phosphorylation and activation of PLCγ and an increase in intracellular calcium upon treatment of HUVEC with Sema4D, responses that were necessary for a pro-angiogenic phenotype observed in vitro. Taken together, these results suggest that Plexin-B1 promotes angiogenesis in endothelial cells by signaling through PI(4)P5Kα and generating lipid second messengers.     

Protein kinase-A inhibits phospholipase-C activity and alters protein phosphorylation in rat myometrial plasma membranes.

Our previous studies implicated the involvement of protein kinase-A in the inhibitory effects of isoproterenol and relaxin on oxytocin-stimulated phosphoinositide turnover in rat myometrium. To understand the possible mechanisms involved, the properties and regulation of phospholipase-C (PLC) in purified myometrial plasma membranes from estrogen-primed rats were studied. The PLC activity measured with exogenous [3H]phosphatidylinositol 4,5-bisphosphate as substrate was Ca2+ dependent. The nonhydrolyzable GTP analog guanosine 5'-(3-O-thio)triphosphate stimulated PLC activity with a ED50 of 1.6 microM and shifted the calcium dependence curve to the left. Guanosine 5'-(3-O-thio)triphosphate-stimulated phosphatidylinositol 4,5-bisphosphate hydrolysis was inhibited by activation of endogenous and exogenous cAMP-dependent protein kinase (PKA). The effects of endogenous and exogenous PKA were significantly reversed by IP20, a potent synthetic peptide inhibitor of PKA. In the presence of [gamma-32Pi]ATP and exogenous PKA, 32Pi was incorporated in an IP20-sensitive manner into major bands at approximately 17,000, 20,000-24,000, 33,000, 38,000, 40,000-44,000, and other higher mol wt. These data indicate that one or more GTP-binding proteins mediate activation of membrane-bound PLC in rat myometrium. Phosphorylation of one or more membrane-associated proteins by PKA may regulate myometrial PLC activity and play a role in the inhibitory effects of isoproterenol and relaxin.     

Polylysine-containing peptides, including the carboxyl-terminal segment of the human c-Ki-ras 2 protein, affect the activity of some key membrane enzymes.

Polylysine-containing peptides are found to affect membrane protein kinases, phosphatidylinositol kinases, and adenylate cyclase. Poly(L-lysine), poly(D-lysine), random copolymers of lysine and serine or lysine and alanine, and poly(L-ornithine) produced large increases in the in vitro phosphorylation of some membrane proteins present in Xenopus laevis oocyte membranes. Poly(L-arginine) did not cause a similar stimulation. In these membranes the phosphorylation of polydisperse protein of approximately 25 kDa was also greatly increased by 1 mM spermine and spermidine, by 10 microM histone H1, or by 200 microM peptide containing the 14-residue sequence at the carboxyl terminus of the human c-Ki-ras 2 gene product, which has eight lysines. Similar specific stimulation of protein phosphorylation was observed with membranes of NG-108-15 nerve cells in culture. Polylysine peptides, including the c-Ki-ras 2 segment, also stimulate the in vitro phosphorylation of membrane inositolphospholipids, to produce mainly phosphatidylinositol 4-phosphate and less phosphatidylinositol 4,5-bisphosphate. Polylysine also alters the activity of oocyte adenylate cyclase, assayed in the presence of either F- or 5'-guanylyl imidodiphosphate.     

p38 mitogen-activated protein kinase contributes to cell cycle regulation by cAMP in FRTL-5 thyroid cells

OBJECTIVE: Thyrotropin activates the cAMP pathway in thyroid cells, and stimulates cell cycle progression in cooperation with insulin or insulin-like growth factor-I. Because p38 mitogen-activated protein kinases (p38 MAPKs) were stimulated by cAMP in the FRTL-5 rat thyroid cell line, we investigated (i) the effect of the specific inhibition of p38 MAPKs on FRTL-5 cell proliferation and (ii) the mechanism of action of p38 MAPKs on cell cycle control, by studying the expression and/or the activity of several cell cycle regulatory proteins in FRTL-5 cells. METHODS: DNA synthesis was monitored by incorporation of [(3)H]thymidine into DNA and the cell cycle distribution was assessed by fluorescence-activated cell sorter analysis. Expression of cell cycle regulatory proteins was determined by Western blot analysis. Cyclin-dependent kinase 2 (Cdk2) activity associated to cyclin E was immunoprecipitated and was measured by an in vitro kinase assay. RESULTS: SB203580, an inhibitor of alpha and beta isoforms of p38 MAPKs, but not its inactive analog SB202474, inhibited DNA synthesis and the G1-S transition induced by forskolin plus insulin. SB203580 inhibited specifically p38 MAPK activity but not other kinase activities such as Akt and p70-S6 kinase. Treatment of FRTL-5 cells with SB203580 decreased total and cyclin E-associated Cdk2 kinase activity stimulated with forskolin and insulin. However, inhibition of p38 MAPKs by SB203580 was without effect on total cyclin E and Cdk2 levels. The decrease in Cdk2 kinase activity caused by SB203580 treatment was not due to an increased expression of p21(Cip1) or p27(Kip1) inhibitory proteins. In addition, SB203580 affected neither Cdc25A phosphatase expression nor Cdk2 Tyr-15 phosphorylation. Inhibition of p38 MAPKs decreased Cdk2-cyclin E activation by regulating the subcellular localization of Cdk2 and its phosphorylation on Thr-160. CONCLUSIONS: These results indicate that p38 MAPK activity is involved in the regulation of cell cycle progression in FRTL-5 thyroid cells, at least in part by increasing nuclear Cdk2 activity.     

Diacylglycerol kinase epsilon regulates seizure susceptibility and long-term potentiation through arachidonoyl- inositol lipid signaling

Arachidonoyldiacylglycerol (20:4-DAG) is a second messenger derived from phosphatidylinositol 4,5-bisphosphate and generated by stimulation of glutamate metabotropic receptors linked to G proteins and activation of phospholipase C. 20:4-DAG signaling is terminated by its phosphorylation to phosphatidic acid, catalyzed by diacylglycerol kinase (DGK). We have cloned the murine DGKepsilon gene that showed, when expressed in COS-7 cells, selectivity for 20:4-DAG. The significance of DGKepsilon in synaptic function was investigated in mice with targeted disruption of the DGKepsilon. DGKepsilon(-/-) mice showed a higher resistance to electroconvulsive shock with shorter tonic seizures and faster recovery than DGKepsilon(+/+) mice. The phosphatidylinositol 4,5-bisphosphate-signaling pathway in cerebral cortex was greatly affected, leading to lower accumulation of 20:4-DAG and free 20:4. Also, long-term potentiation was attenuated in perforant path-dentate granular cell synapses. We propose that DGKepsilon contributes to modulate neuronal signaling pathways linked to synaptic activity, neuronal plasticity, and epileptogenesis.     

Synaptojanin 1-linked phosphoinositide dyshomeostasis and cognitive deficits in mouse models of Down's syndrome

Phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)] is a signaling phospholipid implicated in a wide variety of cellular functions. At synapses, where normal PtdIns(4,5)P(2) balance is required for proper neurotransmission, the phosphoinositide phosphatase synaptojanin 1 is a key regulator of its metabolism. The underlying gene, SYNJ1, maps to human chromosome 21 and is thus a candidate for involvement in Down's syndrome (DS), a complex disorder resulting from the overexpression of trisomic genes. Here, we show that PtdIns(4,5)P(2) metabolism is altered in the brain of Ts65Dn mice, the most commonly used model of DS. This defect is rescued by restoring Synj1 to disomy in Ts65Dn mice and is recapitulated in transgenic mice overexpressing Synj1 from BAC constructs. These transgenic mice also exhibit deficits in performance of the Morris water maze task, suggesting that PtdIns(4,5)P(2) dyshomeostasis caused by gene dosage imbalance for Synj1 may contribute to brain dysfunction and cognitive disabilities in DS.     

Pertussis toxin inhibits fMet-Leu-Phe- but not phorbol ester-stimulated changes in rabbit neutrophils: role of G proteins in excitation response coupling.

Addition of pertussis toxin to rabbit neutrophils inhibits the fMet-Leu-Phe- induced increases in Na+ influx and in intracellular pH. In addition, pretreatment of the cells with the toxin inhibits the decrease in the levels of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate and the enhanced production of phosphatidic acid produced by the chemotactic factor fMet-Leu-Phe. Furthermore, the fMet-Leu-Phe-induced changes in the phosphorylation of a 46-kDa protein and of several other proteins are also inhibited by the toxin. On the other hand, the phorbol 12-myristate 13-acetate (PMA)-induced increases in the phosphorylation of several proteins are not inhibited by the toxin. PMA, but not its inactive analogue 4 alpha-phorbol 12,13-didecanoate, was also found to stimulate Na+ influx and to increase the intracellular pH in rabbit neutrophils. These ionic effects, like those produced by fMet-Leu-Phe, are inhibited by amiloride. The stimulated Na+ influx and H+ efflux produced by the phorbol ester, on the other hand, are not inhibited by pertussis toxin. The results reported here suggest that the activity of the Na+/H+ antiport in neutrophils is regulated by protein kinase C; that the G-protein system, either directly or indirectly, is involved in the stimulus-response coupling sequence in these cells; and that the toxin acts at, or prior to, the steps responsible for the activation of phospholipase C, and it does not affect the sequence of reactions initiated by the activation of the protein kinase C.     

Inhibition of Phosphatidylinositol 4-phosphate 5-kinase by Cyclic AMP in Human Platelets

The effects of cyclic AMP (cAMP) on phosphatidylinositol 4,5-bisphosphate (PI 4,5-P(2)) synthesis were examined in human platelets. In (32)P-prelabeled intact platelets, although the level of [(32)P]phosphatidylinositol 4-phosphate (PI 4-P) was increased by forskolin and prostaglandin-I(2) (PGI(2)), the formation of [(32)P]PI 4,5-P(2) time-and concentration-dependently decreased, suggesting inhibition of phosphatidylinositol 4-phosphate 5-kinase (PI 4-P 5-kinase). In saponin-permeabilized platelets, formation of PI 4-P and PI 4,5-P(2) can be measured by utilizing [γ-(32)P] ATP. In this system, PGI(2) and cAMP inhibited the generation of [(32)P)PI 4,5-P(2). The PI 4-P 5-kinase activity was mostly located in the platelet membrane fraction and was inhibited by cAMP; H-8 and H-89, inhibitors of cAMP-dependent protein kinase (PKA), abolished this inhibitory effect, suggesting that cAMP exerted its action on PI 4-P 5-kinase via PKA. Adenosine, which is reported to directly inhibit phosphatidylinositol 4-kinase (PI 4-kinase) in some types of cells, had no effect on platelet membrane PI 4-P 5-kinase activity. In dbc AMP-pretreated membranes, PI 4-P 5-kinase activity was lower than that of control membranes. The involvement of PKA with the inhibitory action of cAMP in PI 4-P 5-kinase activity was further confirmed using the catalytic subunit of PKA. The synthesis of [(32)P] PI 4,5-P(2) in permeabilized platelets and the specific activity of partially purified PI 4-P 5-kinase were decreased by incubation with the PKA catalytic subunit. The present results indicate that the cAMP-PKA system inhibits PI 4-P 5-kinase activity, leading to decreased formation of PI 4,5-P(2) in human platelets.     

The B'/PR72 subunit mediates Ca2+-dependent dephosphorylation of DARPP-32 by protein phosphatase 2A

In dopaminoceptive neurons, dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) plays a central role in integrating the effects of dopamine and other neurotransmitters. Phosphorylation of DARPP-32 at Thr-34 by protein kinase A results in inhibition of protein phosphatase 1 (PP1), and phosphorylation at Thr-75 by Cdk5 (cyclin-dependent kinase 5) results in inhibition of protein kinase A. Dephosphorylation at Thr-34 involves primarily the Ca(2+)-dependent protein phosphatase, PP2B (calcineurin), whereas dephosphorylation of Thr-75 involves primarily PP2A, the latter being subject to control by both cAMP- and Ca(2+)-dependent regulatory mechanisms. In the present study, we have investigated the mechanism of Ca(2+)-dependent regulation of Thr-75 by PP2A. We show that the PR72 (or B' or PPP2R3A) regulatory subunit of PP2A is highly expressed in striatum. Through the use of overexpression and down-regulation by using RNAi, we show that PP2A, in a heterotrimeric complex with the PR72 subunit, mediates Ca(2+)-dependent dephosphorylation at Thr-75 of DARPP-32. The PR72 subunit contains two Ca(2+) binding sites formed by E and F helices (EF-hands 1 and 2), and we show that the former is necessary for the ability of PP2A activity to be regulated by Ca(2+), both in vitro and in vivo. Our studies also indicate that the PR72-containing form of PP2A is necessary for the ability of glutamate acting at alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and NMDA receptors to regulate Thr-75 dephosphorylation. These studies further our understanding of the complex signal transduction pathways that regulate DARPP-32. In addition, our studies reveal an alternative intracellular mechanism whereby Ca(2+) can activate serine/threonine phosphatase activity.     

Temporal control of the dephosphorylation of Cdk substrates by mitotic exit pathways in budding yeast

The temporal phosphorylation of cell cycle-related proteins by cyclin-dependent kinases (Cdks) is critical for the correct order of cell cycle events. In budding yeast, CDC28 encodes the only Cdk and its association with various cyclins governs the temporal phosphorylation of Cdk substrates. S-phase Cdk substrates are phosphorylated earlier than mitotic Cdk substrates, which ensures the sequential order of DNA synthesis and mitosis. However, it remains unclear whether Cdk substrates are dephosphorylated in temporally distinct windows. Cdc14 is a conserved protein phosphatase responsible for the dephosphorylation of Cdk substrates. In budding yeast, FEAR (Cdc14 early anaphase release) and MEN (mitotic exit network) activate phosphatase Cdc14 by promoting its release from the nucleolus in early and late anaphase, respectively. Here, we show that the sequential Cdc14 release and the distinct degradation timing of different cyclins provides the molecular basis for the differential dephosphorylation windows of S-phase and mitotic cyclin substrates. Our data also indicate that FEAR-induced dephosphorylation of S-phase Cdk substrates facilitates anaphase progression, revealing an extra layer of mitotic regulation.     

Aberrant motor axon projection, acetylcholine receptor clustering, and neurotransmission in cyclin-dependent kinase 5 null mice

Cyclin-dependent kinase (Cdk)5 is a key regulator of neural development. We have previously demonstrated that Cdk5/p35 are localized to the postsynaptic muscle and are implicated in the regulation of neuregulin/ErbB signaling in myotube culture. To further elucidate whether Cdk5 activity contributes to neuromuscular junction (NMJ) development in vivo, the NMJ of Cdk5-/- mice was examined. Consistent with our previous demonstration that Cdk5 phosphorylates ErbB2/3 to regulate its tyrosine phosphorylation, we report here that the phosphorylation of ErbB2 and ErbB3 and the ErbB2 kinase activity are reduced in Cdk5-deficient muscle. In addition, Cdk5-/- mice also display morphological abnormalities at the NMJ pre- and postsynaptically. Whereas the outgrowth of the main nerve trunk is grossly normal, the intramuscular nerve projections exhibit profuse and anomalous branching patterns in the Cdk5-/- embryos. The central band of acetylcholine receptor (AChR) clusters is also wider in Cdk5-/- diaphragms, together with the absence of S100 immunoreactivity along the phrenic nerve during late embryonic stages. Moreover, we unexpectedly discovered that the agrin-induced formation of large AChR clusters is significantly increased in primary muscle cultures prepared from Cdk5-null mice and in C2C12 myotubes when Cdk5 activity was suppressed. These abnormalities are accompanied by elevated frequency of miniature endplate potentials in Cdk5-null diaphragm. Taken together, our findings reveal the essential role of Cdk5 in regulating the development of motor axons and neuromuscular synapses in vivo.     

A voltage-sensing phosphatase, Ci-VSP, which shares sequence identity with PTEN, dephosphorylates phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol lipids play diverse physiological roles, and their concentrations are tightly regulated by various kinases and phosphatases. The enzymatic activity of Ciona intestinalis voltage sensor-containing phosphatase (Ci-VSP), recently identified as a member of the PTEN (phosphatase and tensin homolog deleted on chromosome 10) family of phosphatidylinositol phosphatases, is regulated by its own voltage-sensor domain in a voltage-dependent manner. However, a detailed mechanism of Ci-VSP regulation and its substrate specificity remain unknown. Here we determined the in vitro substrate specificity of Ci-VSP by measuring the phosphoinositide phosphatase activity of the Ci-VSP cytoplasmic phosphatase domain. Despite the high degree of identity shared between the active sites of PTEN and Ci-VSP, Ci-VSP dephosphorylates not only the PTEN substrate, phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3], but also, unlike PTEN, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. Enzymatic action on PI(4,5)P2 removes the phosphate at position 5 of the inositol ring, resulting in the production of phosphatidylinositol 4-phosphate [PI(4)P]. The active site Cys-X(5)-Arg (CX(5)R) sequence of Ci-VSP differs with that of PTEN only at amino acid 365 where a glycine residue in Ci-VSP is replaced by an alanine in PTEN. Ci-VSP with a G365A mutation no longer dephosphorylates PI(4,5)P2 and is not capable of inducing depolarization-dependent rundown of a PI(4,5)P2-dependent potassium channel. These results indicate that Ci-VSP is a PI(3,4,5)P3/PI(4,5)P2 phosphatase that uniquely functions in the voltage-dependent regulation of ion channels through regulation of PI(4,5)P2 levels.     

Cyclin-dependent kinase 5 modulates nociceptive signaling through direct phosphorylation of transient receptor potential vanilloid 1

Transient receptor potential vanilloid 1 (TRPV1), a ligand-gated cation channel highly expressed in small-diameter sensory neurons, is activated by heat, protons, and capsaicin. The phosphorylation of TRPV1 provides a versatile regulation of intracellular calcium levels and is critical for TRPV1 function in responding to a pain stimulus. We have previously reported that cyclin-dependent kinase 5 (Cdk5) activity regulates nociceptive signaling. In this article we report that the Cdk5-mediated phosphorylation of TRPV1 at threonine-407 can modulate agonist-induced calcium influx. Inhibition of Cdk5 activity in cultured dorsal root ganglia neurons resulted in a significant reduction of TRPV1-mediated calcium influx, and this effect could be reversed by restoring Cdk5 activity. Primary nociceptor-specific Cdk5 conditional-knockout mice showed reduced TRPV1 phosphorylation, resulting in significant hypoalgesia. Thus, the present study indicates that Cdk5-mediated TRPV1 phosphorylation is important in the regulation of pain signaling.     

Cyclin-dependent kinase 5 regulates steroidogenic acute regulatory protein and androgen production in mouse Leydig cells

The roles of cyclin-dependent kinase 5 (Cdk5) in central nervous system and neurodegenerative diseases have been intensely investigated in recent decades. Because protein expressions of Cdk5 and its regulator, p35, have been identified in Leydig cells, it is informative to further explore the novel function of Cdk5/p35 in male reproduction. Here we show that Cdk5/p35 protein expression and kinase activity in mouse Leydig cells are regulated by human chorionic gonadotrophin (hCG) in both dose- and time-dependent manners. Blocking of Cdk5 by molecular inhibitors or small interfering RNA resulted in reduction of testosterone production by Leydig cells. cAMP, a second messenger in LH signaling, was identified as a factor in hCG-dependent regulation of Cdk5/p35. Importantly, Cdk5 protein and kinase activity could support accumulation of steroidogenic acute regulatory (StAR) protein, a crucial component of steroidogenesis. We additionally addressed the protein interaction between Cdk5/p35 and StAR. The Cdk5-dependent serine phosphorylation of StAR indicated a possible mechanism by which Cdk5 induced accumulation of StAR protein. In conclusion, Cdk5 modulates hCG-induced androgen production in mouse Leydig cells, possibly through regulation of StAR protein levels. These results indicate that Cdk5 may play an important role in male reproductive endocrinology and is a potential therapeutic target in androgen-related diseases.     

Immunosuppressant FK506 enhances phosphorylation of nitric oxide synthase and protects against glutamate neurotoxicity.

Immunosuppressants FK506 and cyclosporin A inhibit neurotoxicity of N-methyl-D-aspartate in primary cortical cultures, while having no effect on quisqualate- and kainate-mediated neurotoxicity. Rapamycin completely reverses the neuroprotective effect of FK506. Both FK506 and cyclosporin A inhibit NMDA-elicited/nitric oxide-mediated increases in cGMP levels in cortical cultures. FK506 has no effect on sodium nitroprusside-induced increases in cGMP. In a stably transfected human kidney 293 cell line overexpressing the gene encoding nitric oxide synthase [L-arginine, NADPH:oxygen oxidoreductase (nitric oxide-forming), EC 1.14.13.39], FK506 inhibits the calcium ionophore A23187, stimulated increases in nitrite (a breakdown product of nitric oxide), and potentiates phorbol ester-mediated inhibition of nitrite formation. FK506-mediated inhibition of nitric oxide formation is completely reversed by rapamycin. Calcineurin dephosphorylates protein kinase C-mediated phosphorylation of nitric oxide synthase. FK506 prevents the calcineurin-mediated dephosphorylation of nitric oxide synthase and thereby diminishes the enzyme''s catalytic activity. These data establish nitric oxide synthase as a calcineurin substrate. Nitric oxide synthase catalytic activity is regulated by the phosphorylation state of the enzyme. Enhanced phosphorylation of nitric oxide synthase diminishes catalytic activity, and dephosphorylation (through activation of calcineurin) enhances catalytic activity. The neuroprotective effect of FK506 and cyclosporin A presumably involves the inhibition of calcineurin, preventing the dephosphorylation of nitric oxide synthase and its subsequent activation.     

The SCFCdc4 ubiquitin ligase regulates calcineurin signaling through degradation of phosphorylated Rcn1, an inhibitor of calcineurin

The highly conserved RCN family of proteins regulates the serine/threonine protein phosphatase calcineurin, which is required for the expression of genes involved in Ca(2+)-dependent processes, such as the control of memory, apoptosis, T cell activation, cell cycle, Ca(2+)-homeostasis, and skeletal and cardiac muscle growth and differentiation. However, RCNs regulate calcineurin through two paradoxical actions: they act as feedback inhibitors of calcineurin, whereas their phosphorylation stimulates calcineurin. Here we show that phosphorylation of yeast RCN, Rcn1, triggers degradation through the SCF(Cdc4) ubiquitin ligase complex. Degradation of phosphorylated Rcn1 is required to mitigate inhibition of calcineurin by Rcn1 and results in activation of calcineurin activity in response to Ca(2+) as well as in reactivation of calcineurin in response to changes in Ca(2+) concentration. The SCF(Cdc4)-dependent degradation required phosphorylation of Rcn1 by Mck1, a member of the GSK3 family of protein kinases, and was promoted by Ca(2+). However, such degradation was counteracted by dephosphorylation of Rcn1, which was promoted by Ca(2+)-stimulated calcineurin. Thus, calcineurin activity is fine-tuned to Ca(2+) signals by mechanisms that have opposite functions. Our results identify the molecular mechanism of Rcn1 phosphorylation-induced stimulation of the phosphatase activity of calcineurin. The results provide insight into the mechanism involved in maintaining proper responses to Ca(2+) signals.     

Chlorpromazine inhibits vesiculation, alters phosphoinositide turnover and changes deformability of ATP-depleted RBCs

To delineate further the underlying mechanism by which amphiphilic drugs can modulate vesicle release from human RBCs, we studied the effect of chlorpromazine on erythrocyte vesiculation induced by ATP depletion. This was correlated with turnover of the phosphoinositides as well as RBC deformability during the process since phosphoinositide metabolism may be involved in shape regulation of RBCs. Echinocytic shape transformation and subsequent vesiculation of RBCs, which commonly occur during ATP depletion, were inhibited by chlorpromazine. Furthermore, with a newly developed two-dimensional thin-layer chromatography separation of RBC membrane phospholipids, we showed that chlorpromazine significantly decreased the dephosphorylation of phosphatidylinositol-4,5-bisphosphate (PIP2) in both ATP-depleted RBCs as well as in cells with partly maintained ATP levels. Concomitantly, there was a smaller increase in the relative amount of phosphatidylinositol. In addition, chlorpromazine also inhibited the decreased in RBC deformability as well as the shift of osmotic fragility that occurs during ATP depletion of erythrocytes.