Time Course Of Phosphorylation Research Articles

Rho-kinase-mediated regulation of receptor-agonist-stimulated smooth muscle contraction

Rho kinase was shown to regulate smooth muscle contraction through modulating myosin phosphatase (MLCP) activity, but the in vivo mechanism remains to be clarified. This study examined the effects of Rho kinase inhibition on the phosphorylation time course of MLCP subunit MYPT1 at Thr697 and Thr855 and MLCP inhibitory protein CPI-17 at Thr38 and on actin polymerization during the contraction of rat tail artery (RTA) smooth muscle. Rho kinase inhibitor Y27632 suppressed force activated by alpha(1)-adrenergic agonist phenylephrine or thromboxane A(2) analog U46619 with concomitant decreases in MLC(20) phosphorylation. Phenylephrine and U46619 significantly increased MYPT1(Thr855) phosphorylation that was eliminated by Y27632 pretreatment, whereas MYPT1(Thr697) phosphorylation was not stimulated. Phenylephrine increased CPI-17(Thr38) phosphorylation that was not inhibited by Y27632 but was abolished by a protein kinase C inhibitor Ro 31-8220; in contrast, U46619 did not stimulate CPI-17 phosphorylation. Both agonists increased actin polymerization that was diminished by Y27632 under phenylephrine but not U46619 activation. These results demonstrated a temporal correlation between MYPT1(Thr855) phosphorylation, MLC(20) phosphorylation, and contraction in a Rho-kinase-dependent manner for both phenylephrine and U46619 stimulation, suggesting that Rho kinase regulates MLCP activity through MYPT1(Thr855) phosphorylation during RTA smooth muscle contraction. Furthermore, Rho kinase regulates actin polymerization activated by alpha(1)-adrenoceptors but is less significant in thromboxane receptor stimulation.

Astrocyte-conditioned medium protecting hippocampal neurons in primary cultures against corticosterone-induced damages via PI3-K/Akt signal pathway

Prolonged or excessive exposure to corticosterone leads to neuronal damages in the brain regions, including hippocampus. We reported that astrocyte-conditioned medium (ACM) protected the neurons of the primary hippocampal cultures against the corticosterone-induced damages. Corticosterone added to the cultures resulted in a significant number of TUNEL-positive cells. However, corticosterone-induced TUNEL labeling was suppressed as for ACM-cultured neurons. To delineate the molecular basis underlying the neuroprotection of ACM, we assessed the activation of ERK1/2 and (PI3-K)/Akt signal pathways in response to corticosterone-induced neuronal damages. Western blot test revealed that corticosterone increased the phosphorylation of ERK1/2 and PI3-K/Akt in hippocampal neurons grown in Neurobasal medium supplemented with B27 and 500 μm l-glutamine (NBM +). Interestingly, the increase of phospho-ERK1/2 and Akt levels was much pronounced and the time course of phosphorylation was altered in ACM, suggesting that both signaling pathways might participate in ACM protection. Furthermore, the selective inhibitor of Akt, rather than ERK1/2, blocked the neuroprotective activity against corticosterone in ACM-cultured neurons. In summary, our data showed that ACM had a potent neuroprotective effect in cultured neurons. PI3-K/Akt signal pathway, but not ERK1/2, was involved in the protective activity against the corticosterone-induced damages.

Dissection of the Functional Differences between Human Secretory Pathway Ca2+/Mn2+-ATPase (SPCA) 1 and 2 Isoenzymes by Steady-state and Transient Kinetic Analyses

Human secretory pathway Ca2+/Mn2+-ATPase (SPCA) 2 encoded by ATP2C2 is only expressed in a limited number of tissues, unlike the ubiquitously expressed SPCA1 pump (encoded by ATP2C1, the gene defective in Hailey-Hailey disease). It has not been determined whether there are significant functional differences between SPCA1 and SPCA2 pump enzymes. Therefore, steady-state and transient kinetic approaches were used to characterize the overall and partial reactions of the Ca2+ transport cycle mediated by the human SPCA2 enzyme upon heterologous expression in HEK-293 cells. The catalytic turnover rate of SPCA2 was found enhanced relative to SPCA1 pumps. SPCA2 displayed a very high apparent affinity for cytosolic Ca2+ (K0.5 = 0.025 microm) in activation of the phosphorylation activity but still 2.5-fold lower than that of SPCA1d. Our kinetic analysis traced both differences to the increased rate characterizing the E1 approximately PCa to E2-P transition of SPCA2. Moreover, the reduced rate of the E2 to E1 transition seems to contribute in determining the lower apparent Ca2+ affinity and the increased sensitivity to thapsigargin inhibition, relative to SPCA1d. SPCA2 also displayed a reduced apparent affinity for inorganic phosphate, which could be explained by the observed enhanced rate of the E2-P dephosphorylation. The insensitivity to modulation by pH and K+ concentration of the constitutively enhanced E2-P dephosphorylation of SPCA2 is similar to SPCA1d and possibly represents a novel SPCA-specific feature, which is not shared by sarco(endo)plasmic reticulum Ca2+-ATPases.

Disease-related Modifications in Tau Affect the Interaction between Fyn and Tau

Microtubule-associated protein tau is the major component of the neurofibrillary tangles of Alzheimer disease (AD) and is genetically linked to frontotemporal dementias (FTDP-17). We have recently shown that tau interacts with the SH3 domain of Fyn, an Src family non-receptor tyrosine kinase, and is tyrosine-phosphorylated by Fyn on Tyr-18. Also, tyrosine-phosphorylated tau is present in the neuropathology of AD. To determine whether alterations in the tau-Fyn interaction might correlate with disease-related factors in AD and FTDP-17, we have performed real-time surface plasmon resonance studies on a panel of 21 tau constructs with Fyn SH3. We report that the interaction between Fyn SH3 and 3R-tau was 20-fold higher than that with 4R-tau. In addition, the affinity between 4R-tau and Fyn SH3 was increased 25-45-fold by phosphorylation-mimicking mutations or by FTDP-17 mutations. In vitro kinase reactions show that tau, with lower affinity SH3 interactions, exhibited a lower level of Tyr-18 phosphorylation under our reaction conditions. Lastly, we have demonstrated that tau is phosphorylated on Tyr-18 in the tau P301L mouse model for tauopathy (JNPL3). In summary, our results suggest that disease-related phosphorylation and missense mutations of tau increase association of tau with Fyn. Because these effects are mediated through the 4R component of the tau population, these results also have implications for the FTDP-17 diseases caused by increased expression of 4R-tau. Our data support a role for the Fyn-tau interaction in neurodegeneration.

Both Saturated and n-6 Polyunsaturated Fat Diets Reduce Phosphorylation of Insulin Receptor Substrate-1 and Protein Kinase B in Muscle during the Initial Stages of in Vivo Insulin Stimulation

Our aim was to determine the importance of changes in phosphorylation of key insulin signaling intermediates in the insulin resistance observed in skeletal muscle of rats fed diets high in saturated or n-6 polyunsaturated fat. We used phospho-specific antibodies to measure the time course of phosphorylation of key components of the insulin signaling pathway by immunoblotting during the initial stages of a physiological elevation in the circulating insulin concentration. The phosphorylation of insulin receptor at Tyr1162/1163 (IR Tyr1162/1163) increased over 20 min of insulin infusion, whereas the downstream phosphorylation of insulin receptor substrate-1 Tyr612 (IRS-1 Tyr612) peaked at 5 min and declined thereafter. Interestingly, phosphorylation of IRS-1 at Tyr895 continued to increase over the 20-min period, and protein kinase B (PKB) phosphorylation at Ser473 reached a plateau by 5 min, demonstrating that different profiles of phosphorylation are involved in transmission of the insulin signal despite a constant level of insulin stimulation. In muscle from rats fed high n-6 polyunsaturated or saturated fat diets, however, there was no insulin-stimulated increase in IRS-1 Tyr612 phosphorylation and a temporal difference in PKB Ser473 phosphorylation despite no difference in IR Tyr1162/1163 phosphorylation, IRS-1 Tyr895 phosphorylation, and ERK phosphorylation. These results demonstrate that under conditions of increased insulin, similar to those used to assess insulin action in vivo, chronic high-fat feeding impairs insulin signal transduction related to glucose metabolism at the level of IRS-1 Tyr612 and PKB Ser473 and that these effects are independent of the type of fat used in the high-fat diet.

Phosphorylation of Human p53 at Serine 46 Determines Promoter Selection and whether Apoptosis Is Attenuated or Amplified

The capacity of DNA damaging agents to induce apoptosis is regulated by target gene induction by p53. We found that p53 targeted MDM2 in cells in which DNA repair was occurring, but persistent DNA damage induced by chemotherapy led p53 to selectively target PTEN. High dose chemotherapy induced the phosphorylation of p53 on serine 46, whereas low dose chemotherapy did not. A nonphosphorylatable serine 46 to alanine p53 mutant (S46A) targeted the MDM2 promoter in preference to that for PTEN. A serine 46 to aspartate mutant (S46D, a phosphorylation mimic) targeted PTEN in preference to MDM2. These observations show that phosphorylation of serine 46 in p53 is sufficient for it to induce the PTEN (phosphatase and tensin homolog deleted on chromosome ten) tumor suppressor protein in preference to MDM2. S46A induced significantly less cell death than the S46D in cells. The phosphorylation-induced change of p53 promoter targeting suppresses the induction of MDM2 and the formation of the autoregulatory feedback loop. Induction of PTEN by p53 followed by expression of PTEN inhibits AKT-induced translocation of MDM2 into the nucleus and sustains p53 function. The protection of p53 from MDM2 by PTEN and the damage-induced activation of PTEN by phosphorylated p53 leads to the formation of an apoptotic amplification cycle in which p53 and PTEN coordinately increase cellular apoptosis.

Identification of Phosphopeptides by MALDI Q-TOF MS in Positive and Negative Ion Modes after Methyl Esterification

We have developed an efficient, sensitive, and specific method for the detection of phosphopeptides present in peptide mixtures by MALDI Q-TOF mass spectrometry. Use of the MALDI Q-TOF enables selection of phosphopeptides and characterization by CID of the phosphopeptides performed on the same sample spot. However, this type of experiment has been limited by low ionization efficiency of phosphopeptides in positive ion mode while selecting precursor ions of phosphopeptides. Our method entails neutralizing negative charges on acidic groups of nonphosphorylated peptides by methyl esterification before mass spectrometry in positive and negative ion modes. Methyl esterification significantly increases the relative signal intensity generated by phosphopeptides in negative ion mode compared with positive ion mode and greatly increases selectivity for phosphopeptides by suppressing the signal intensity generated by acidic peptides in negative ion mode. We used the method to identify 12 phosphopeptides containing 22 phosphorylation sites from low femtomolar amounts of a tryptic digest of beta-casein and alpha-s-casein. We also identified 10 phosphopeptides containing five phosphorylation sites from an in-gel tryptic digest of 100 fmol of an in vitro autophosphorylated fibroblast growth factor receptor kinase domain and an additional phosphopeptide containing another phosphorylation site when 500 fmol of the digest was examined. The results demonstrate that the method is a fast, robust, and sensitive means of characterizing phosphopeptides present in low abundance mixtures of phosphorylated and nonphosphorylated peptides.

Role of phosphorylation of Thr residue of phospholamban in mechanical recovery during hypercapnic acidosis

To assess the time course of phosphorylation of phospholamban residues, the underlying mechanisms determining these phosphorylations, and their functional impact on the mechanical recovery during acidosis. Langendorff perfused rat hearts were submitted to 30 min of hypercapnic acidosis. Contractility, relaxation, and phosphorylation of phospholamban residues, immunodetected by specific antibodies, were determined. Acidosis produced a mechanical impairment followed by a spontaneous recovery, most of which occurred within the first 3 min of acidosis (early recovery). During this period, contractility and relaxation recovered by 67+/-9% and 77+/-11%, respectively, from its maximal depression, together with an increase in the Ca(2+)-calmodulin-dependent protein kinase II (CaMKII)-dependent phosphorylation of Thr(17). The CaMKII inhibitor KN-93, at 1, 5 and 10 microM, decreased Thr(17) phosphorylation to basal levels and produced a similar impairment of the early relaxation recovery (50%). However, only 5 and 10 microM KN-93 inhibited the early contractile recovery and completely blunted the late mechanical recovery. Inhibition of the reverse mode of the Na(+)/Ca(2+) exchanger by KB-R7943 decreased Thr(17) phosphorylation but accelerated the early contractile recovery. CaMKII-dependent Thr(17) phosphorylation significantly increased at the beginning of acidosis, is responsible for 50% of the early relaxation recovery, and is linked to the activation of the reverse Na(+)/Ca(2+) mode. The early contractile recovery and the late mechanical recovery are dependent on CaMKII but independent of the phosphorylation of the Thr(17) residue of phospholamban. The reverse Na(+)/Ca(2+) mode has an additional negative effect that opposes the early mechanical recovery.

Modulation of Erythrocyte Membrane Mechanical Function by Protein 4.1 Phosphorylation

Erythrocyte membrane mechanical function is regulated by the spectrin-based membrane skeleton composed of alpha- and beta-spectrin, actin, protein 4.1R (4.1R), and adducin. Post-translational modifications of these proteins have been suggested to modulate membrane mechanical function. Indeed, beta-spectrin phosphorylation by casein kinase I has been shown to decrease membrane mechanical stability. However, the effects of the phosphorylation of skeletal proteins by protein kinase C (PKC), a serine/threonine kinase, have not been elucidated. In the present study, we explored the functional consequences of the phosphorylation of 4.1R and adducin by PKC. We identified Ser-312 in 4.1R as the PKC phosphorylation site. Using antibodies raised against phosphopeptides of 4.1R and adducin, we documented significant differences in the time course of phosphorylation of adducin and 4.1R by PKC. Although adducin was phosphorylated rapidly by the activation of membrane-bound atypical PKC by phorbol 12-myristate 13-acetate stimulation, there was a significant delay in the phosphorylation of 4.1R because of delayed recruitment of conventional PKC from cytosol to the membrane. This differential time course in the phosphorylation of 4.1R and adducin in conjunction with membrane mechanical stability measurements enabled us to document that, although phosphorylation of adducin by PKC has little effect on membrane mechanical stability, additional phosphorylation of 4.1R results in a marked decrease in membrane mechanical stability. We further showed that the phosphorylation of 4.1R by PKC results in its decreased ability to form a ternary complex with spectrin and actin as well as dissociation of glycophorin C from the membrane skeleton. These findings have enabled us to define a regulatory role for 4.1R phosphorylation in dynamic regulation of red cell membrane properties.

Phosphorylation and dephosphorylation of calsequestrin on CK2-sensitive sites in heart

Calsequestrin (CSQ) concentrates in junctional sarcoplasmic reticulum (SR) where it functions in regulation of Ca2+ release. When purified from heart tissue, cardiac CSQ contains phosphate on a cluster of C-terminal serine residues, but little is known about the cellular site of kinase action, and the identity of the kinase remains uncertain. To determine basic features of the phosphorylation, we examined the reaction in canine heart preparations. CSQ phosphorylation was observed in [32P]metabolically-labeled heart cells after adenoviral overexpression, and its constitutive phosphorylation was limited to a CK2-sensitive C-terminal serine cluster. The CSQ kinase was oriented intralumenally, as was CSQ, inside membrane vesicles, such that exposure to each required detergent permeabilization. Yet even after detergent permeabilization, CSQ was phosphorylated much less efficiently by protein kinase CK2 in cardiac microsomes than was purified CSQ. Reduced phosphorylation was strongly dependent upon protein concentration, and phosphorylation time courses revealed a phosphatase activity that occurred constitutively as phosphorylated substrate accumulates. Evidence of selective dephosphorylation of CSQ glycoforms in heart homogenates was also seen by mass spectrometry analysis. Molecules with greater mannose content, a feature of early secretory pathway compartments, were more highly phosphorylated, while greater dephosphorylation was apparent in more distal compartments. Taken together, the analyses of CSQ phosphorylation in heart suggest that a constitutive process of phosphate turnover occurs for cardiac CSQ perhaps associated with its intracellular transport.

Effect of intermittent hypoxia on long-term potentiation in rat hippocampal slices

Intermittent hypoxia (IH) during sleep has been shown to induce apoptosis in a time-dependent manner and spatial learning deficits in adult rats. Recently, we have demonstrated that IH induced significant decreases in Ser-133-phosphorylated cAMP-response element-binding protein (pCREB) without changes in total CREB. The expression of cleaved caspase 3 in the hippocampal CA1, a marker of apoptosis, peaked at 3 days of IH and returned to normoxic values at 14 days of IH. In addition, biphasic changes in spatial task learning were correlated with the CREB phosphorylation time course. In the present study, the rat hippocampal slice preparation was used to evaluate the ability to induce and maintain a CA1 population spike long-term potentiation (PS-LTP) in room air (RA)-maintained and IH-exposed rats. A significant decrease in the ability to sustain PS-LTP for 15 min in slices prepared from IH-exposed rats for either 3 days (34% of total) or 7 days (51% of total) as compared to slices prepared from RA-maintained rats (76% of total) was observed. These results suggest that the diminishment in the ability of neuronal tissue to express and sustain PS-LTP is correlated with previously reported biphasic changes in CREB phosphorylation and programmed cell death.

Human milk induces fetal small intestinal cell proliferation - involvement of a different tyrosine kinase signaling pathway from epidermal growth factor receptor

Breast milk has non-nutritional protective effects on recipient infants. It has been speculated that bioactive substances present in human milk have important roles in protecting infants. However, the mechanisms by which such substances protect newborns are unclear. Therefore, we analyzed the growth-promoting activity of human milk and the intracellular signaling mechanism thereof using human fetal small intestinal (FHS 74 Int) cells. Epidermal growth factor (EGF) stimulated the proliferation of these cells. However, this stimulation was less effective than that of aqueous milk (5% vol/vol). The bioactivity of human milk was heat stable but protease sensitive. EGF receptor tyrosine kinase inhibitor did not repress the milk-induced growth-promoting effect on fetal small intestinal cells. Regarding the intracellular signaling pathway, the milk-induced cell proliferation pathway was tyrosine kinase dependent but was neither mitogen-activated protein (MAP) kinase nor phosphatidylinositol-3 (PI-3) kinase dependent. On the other hand, EGF-induced cell proliferation was tyrosine kinase, MAP kinase, and PI-3 kinase dependent. Rapid tyrosine phosphorylation of several intracellular proteins was detected after milk stimulation. Furthermore, the time course of phosphorylation induced by milk was different from that induced by EGF. The sizes of the proteins phosphorylated in response to milk were different from those of the Shc proteins phosphorylated in response to EGF. These results suggest that human milk induces fetal intestinal cell proliferation through a unique tyrosine kinase pathway different from the EGF receptor signaling pathway.

Thioredoxin and Its Relatives Are Specific Regulators

The formation of reactive oxygen species is implicated in many signaling processes, including the pathways triggered by tumor necrosis factor-α (TNF-α). Thioredoxin (Trx) and thioredoxin-related proteins (TRP) are disulfide reductases that contribute to the redox regulation of cells. In their reduced form, these enzymes can reduce disulfide bonds in substrate proteins. In two articles this week, Jeong et al . characterized the activity of TRP14 and compared the roles of TRP14 and Trx1 in TNF-α signaling using overexpression or depletion by RNA interference. Both proteins inhibit nuclear factor-κB (NF-κB) signaling, as determined by increased Mn 2+ -dependent superoxide dismutase expression (an NF-κB target gene), in response to TNF-α exposure in cells depleted of Trx1 or TRP14. The rate and extent of phosphorylation and degradation of the inhibitor of κB (IκBα) were also increased in Trx1- or TRP14-depleted cells. Depletion of Trx1 or TRP14 also resulted in accelerated caspase cleavage and enhanced cell death in TNF-α-stimulated cells in which protein synthesis was blocked. More interesting were the differences observed after depletion of TRP14 and Trx1. Depletion of Trx1 did not affect the time course or extent of phosphorylation of the two mitogen-activated protein kinase (MAPK) family members JNK and p38, whereas depletion of TRP14 did not alter the time course of phosphorylation but did increase the extent of phosphorylation of these two enzymes. Another difference was the interaction and inhibition of the MAPK kinase kinase ASK1 by Trx1, which did not occur with TRP14. In vitro substrate trapping experiments identified LC8 (the light chain of dynein, which is also known to interact with IκBα and the Bcl-2 member Bim), cofilin, and ribosomal protein L30 as potential substrates of TRP14. In transfected cells, LC8 and TRP14 formed a complex in response to TNF-α or peroxide treatment. Thus, the thioredoxin family of proteins appears to serve specific regulatory roles in redox regulation. W. Jeong, T.-S. Chang, E. S. Boja, H. M. Fales, S. G. Rhee, Roles of TRP14, a thioredoxin-related protein in tumor necrosis factor-α signaling pathways. J. Biol. Chem. 279 , 3151-3159 (2004). [Abstract] [Full Text] W. Jeong, H. W. Yoon, S.-R. Lee, S. G. Rhee, Identification and characterization of TRP14, a thioredoxin-related protein of 14 kDa: new insights into the specificity of thioredoxin function. J. Biol. Chem. 279 , 3142-3150 (2004). [Abstract] [Full Text]

Renal ischemia/reperfusion and ATP depletion/repletion in LLC-PK1 cells result in phosphorylation of FKHR and FKHRL1

Cell death and survival pathways are critical determinants of epithelial cell fate after ischemia. Forkhead proteins have been implicated in the regulation of cellular survival. We have found that none of the forkhead family of proteins, FKHR, is phosphorylated after ischemia/reperfusion in the rat kidney. The time course of phosphorylation is similar to the time course of activation of the forkhead protein kinase Akt/protein kinase B (PKB), with maximal phosphorylation at 24 to 48 hours postreperfusion when the process of regeneration peaks. Extracellular signal-regulated kinase (ERK)1/2 activation has also been implicated as prosurvival in the injured kidney. ERK1/2 were phosphorylated in postischemic kidneys at 5, 30, and 90 minutes of reperfusion, with phosphorylation decreased by 24 and 48 hours. Immunocytochemical analysis revealed increased phospho-ERK1/2 in the thick ascending limb and isolated cells of the S3 segment, which have lost apical actin staining. To understand the relationship between forkhead phosphorylation, Akt, and ERK1/2, an in vitro model of injury was employed. After 40 minutes of chemical anoxia followed by dextrose addition for 20 minutes to replete adenosine triphosphate (ATP) levels, FKHR and FKHRL1 are phosphorylated. The levels of phospho-Akt are increased for at least 120 minutes after dextrose addition with a maximum at 20 minutes. Phosphorylation of Akt, FKHR, and FKHRL1 are phosphatidylinositol 3-kinase (PI 3-kinase) dependent since phosphorylation is reduced by the PI 3-kinase inhibitors, wortmannin, or LY294002. Inhibition of mitogen-activated protein kinase (MAPK)/ERK kinase (MEK1/2), the upstream activator of ERK1/2, has no effect on forkhead protein phosphorylation after chemical anoxia/dextrose addition. We conclude that PI 3-kinase and Akt are activated after renal ischemia/reperfusion and that Akt phosphorylation leads to phosphorylation of FKHR and FKHRL1, which may affect epithelial cell fate in acute renal failure.

Specificity of insulin-like growth factor I and insulin on Shc phosphorylation and Grb2 recruitment in caveolae.

Caveolae are lipid raft microdomains that regulate endocytosis and signal transduction. IGF-I receptor (IGF-IR) localizes in caveolae and tyrosine phosphorylates caveolin 1, supporting a role for these subcellular regions in the compartmentalization of IGF-I signaling. Src homology 2/alpha-collagen related protein (Shc) is the main mediator of IGF-I mitogenic action, coupling IGF-IR phosphorylation to Ras-MAPK activation. Here we show that IGF-I induces Shc tyrosine phosphorylation in the caveolae with a time course significantly different from that observed in the nonraft cellular fractions. In the same time, IGF-I recruits growth factor receptor bound protein 2 (Grb2) to caveolae and activates p42/p44 MAPKs in these microdomains. Src family kinases regulate IGF-I action through an Shc-dependent mechanism. In R-IGF-IRWT cells, IGF-I causes Fyn enrichment in the caveolae with a time course consistent with Shc phosphorylation and Grb2 recruitment in these regions. Finally, we have observed that after IGF-I stimulation, IGF-IR and Fyn colocalize in lipid raft caveolin 1-enriched microdomains. As insulin and IGF-I share common substrates, the effect of insulin on these cellular processes was measured. Here we show that insulin also induces Shc phosphorylation and Grb2 recruitment to caveolae, but with a significantly different time course compared with IGF-I. Our results suggest that 1) IGF-I causes the colocalization of signaling proteins in caveolae through a phosphorylation-regulated mechanism; and 2) the time course of phosphorylation and recruitment of substrates in caveolae by insulin receptor and IGF-IR could determine the specific actions of these receptors.

Delayed phosphorylation of p38 mitogen-activated protein kinase in the AT1a knock-out mouse striatal neurons during middle cerebral artery occlusion and reperfusion

To investigate whether the phosphorylation of p38 in cerebral ischemia occurs via angiotensin II receptor type 1a (AT1a), we examined the time course of phosphorylation of p38 and proline-rich tyrosine kinase 2 in AT1a knock-out mouse striatal neurons during middle cerebral artery occlusion (MCAO) and reperfusion. Phosphorylated-p38 was observed after 2 h and 5 h of reperfusion after 1 h of MCAO in C57/B6 mice and AT1a knock out mice, respectively. We demonstrated a delay of phosphorylation of p38 in the reperfusion model of the AT1a knock-out mouse, and detected microglia in the striatum on the ischemic side that were phosphorylated-p38-positive after 71 h of reperfusion in both animals. However, there was no association between AT1a and delayed neuronal cell death, or between AT1a and activation of caspase-9 in cerebral ischemia/reperfusion.

Pre-steady-state phosphorylation and dephosphorylation of detergent-purified plasma-membrane Ca2+-ATPase.

Pre-steady-state phosphorylation and dephosphorylation of purified and phospholipid-depleted plasma-membrane Ca(2+)-ATPase (PMCA) solubilized in the detergent polyoxyethylene 10 lauryl ether were studied at 25 degrees C. The time course of phosphorylation with ATP of the enzyme associated with Ca(2+), probably the true phosphorylation reaction, showed a fast phase (k(app) near 400 s(-1)) followed by a slow phase (k(app)=23 s(-1)). With asolectin or acidic phosphatidylinositol, the concentration of phosphoenzyme (EP) increased at as high a rate as before, passed through a maximum at 4 ms and stabilized at a steady level that was approx. half that without lipids. Calmodulin (CaM) did not change the rate of the fast phase, accelerated the slow phase (k(app)=93 s(-1)) and increased [EP] with small changes in the shape of the time course. Dephosphorylation was slow (k(app)=30 s(-1)) and insensitive to CaM. Asolectin accelerated dephosphorylation, which followed biexponential kinetics with fast (k(app)=220 s(-1)) and slow (k(app)=20 s(-1)) components. CaM stimulated the fast component by nearly 50%. The results show that the behaviour of the PMCA is complex, and suggest that acidic phospholipids and CaM activate PMCA through different mechanisms. Acceleration of dephosphorylation seems relevant during activation of the PMCA by acidic phospholipids.

Pre-steady-state phosphorylation and dephosphorylation of detergent-purified plasma-membrane Ca2+-ATPase

Pre-steady-state phosphorylation and dephosphorylation of purified and phospholipid-depleted plasma-membrane Ca2+-ATPase (PMCA) solubilized in the detergent polyoxyethylene 10 lauryl ether were studied at 25°C. The time course of phosphorylation with ATP of the enzyme associated with Ca2+, probably the true phosphorylation reaction, showed a fast phase (kapp near 400s−1) followed by a slow phase (kapp = 23s−1). With asolectin or acidic phosphatidylinositol, the concentration of phosphoenzyme (EP) increased at as high a rate as before, passed through a maximum at 4ms and stabilized at a steady level that was approx. half that without lipids. Calmodulin (CaM) did not change the rate of the fast phase, accelerated the slow phase (kapp = 93s−1) and increased [EP] with small changes in the shape of the time course. Dephosphorylation was slow (kapp = 30s−1) and insensitive to CaM. Asolectin accelerated dephosphorylation, which followed biexponential kinetics with fast (kapp = 220s−1) and slow (kapp = 20s−1) components. CaM stimulated the fast component by nearly 50%. The results show that the behaviour of the PMCA is complex, and suggest that acidic phospholipids and CaM activate PMCA through different mechanisms. Acceleration of dephosphorylation seems relevant during activation of the PMCA by acidic phospholipids.

DNA repair protein Rad55 is a terminal substrate of the DNA damage checkpoints.

Checkpoints, which are integral to the cellular response to DNA damage, coordinate transient cell cycle arrest and the induced expression of DNA repair genes after genotoxic stress. DNA repair ensures cellular survival and genomic stability, utilizing a multipathway network. Here we report evidence that the two systems, DNA damage checkpoint control and DNA repair, are directly connected by demonstrating that the Rad55 double-strand break repair protein of the recombinational repair pathway is a terminal substrate of DNA damage and replication block checkpoints. Rad55p was specifically phosphorylated in response to DNA damage induced by the alkylating agent methyl methanesulfonate, dependent on an active DNA damage checkpoint. Rad55p modification was also observed after gamma ray and UV radiation. The rapid time course of phosphorylation and the recombination defects identified in checkpoint-deficient cells are consistent with a role of the DNA damage checkpoint in activating recombinational repair. Rad55p phosphorylation possibly affects the balance between different competing DNA repair pathways.

Exploiting the difference between intrinsic and extrinsic kinases: implications for regulation of signaling by immunoreceptors.

When receptors must interact with an extrinsic kinase to initiate signaling, the kinase can play a regulatory role that is not available to intrinsic receptor kinases. Whether control is exercised at this level depends critically on the amount of kinase available to the receptors and on the potential for redistribution of the kinase during signaling. This study demonstrates that the high affinity receptor for IgE (Fc epsilonRI) on rat basophilic leukemia cells is regulated by its initiating kinase. We present a mathematical model that allows for the reversible recruitment of extrinsic kinases to phosphorylated immunoreceptor tyrosine-based activation motifs. By comparing model predictions to experimental time courses of phosphorylation, we infer that Lyn is limiting, that redistribution occurs after receptors are aggregated, and that the redistribution makes the relationship between tyrosine phosphorylation and receptor aggregation nonlinear.