Calcineurin in the adult rat hippocampus: different distribution in CA1 and CA3 subfields

We examined the immunohistochemical regional distribution of calcineurin (Ca2+/calmodulin-dependent protein phosphatase) in the adult rat hippocampus, following various regional destruction. In the normal adult rat hippocampus, the calcineurin immunoreactivity showed a characteristic pattern. This protein phosphatase was detected in all layers of the CA1 subfield, including the cytoplasm of the pyramidal cells, whereas it was strongly evident in the stratum lucidum and moderately so in the cytoplasm of pyramidal cells in the CA3 subfield. Seven days after transient forebrain ischemia, which induced destruction of CA1 pyramidal cells, the calcineurin immunoreactivity decreased in all layers of the CA1 subfield, while the immunoreactivity for synapsin I, a marker of the presynaptic site, was preserved. Seven days after the intraventricular injection of kainate, which induced destruction of CA3 pyramidal cells, the calcineurin immunoreactivity in the stratum lucidum was preserved, although the immunostaining pattern of the stratum lucidum changed when CA3 pyramidal cells were destroyed. Seven days after mechanical destruction of the dentate gyrus and CA4 subfield, which induced destruction of mossy fibers, the calcineurin immunoreactivity in the stratum lucidum was lost, except in the far site of the stratum lucidum. In the CA1 subfield, calcineurin was mainly located in postsynaptic sites, while it was mainly located in the presynaptic sites in the mossy fibers of the CA3 subfield. The immunohistochemistry of adjacent sections with antibodies of microtubule-associated protein 2 and synapsin I, which are markers of postsynaptic and presynaptic sites respectively, supports these results. Thus, calcineurin has a different synaptical distribution in the rat hippocampus.

Cyclic AMP inhibits activation of mitogen-activated protein kinase and cell proliferation in response to growth factors in cultured rat cortical astrocytes

The cyclic AMP (cAMP)-induced inhibitory effect on cell proliferation was examined through inhibition of mitogen-activated protein kinase (MAP kinase) activation in cultured rat cortical astrocytes. Basic fibroblast growth factor (bFGF) at 10 ng/ml maximally stimulated MAP kinase activity, which peaks during 10 min and prolonged for 24 h. Likewise, DNA synthesis was maximally potentiated with 10 ng/ml bFGF and correlated with MAP kinase activity in a dose-dependent manner. Dibutyryl cAMP (dbcAMP) at 1 mM and isoproterenol at 10 microM inhibited MAP kinase activation and DNA synthesis potentiation with bFGF and platelet-derived growth factor to the control level in cultured astrocytes and C6 glioma cells. The stimulation with bFGF caused a prominent translocation of MAP kinase from the cytosol to the nucleus after 1 h in astrocytes. Treatment of the cells with dbcAMP and isoproterenol completely prevented the translocation of MAP kinase. In experiments with 32P-labeled cultured astrocytes, phosphorylation of Raf-1 was apparently stimulated with bFGF. Treatment with dbcAMP or isoproterenol had a greatly inhibitory effect on the stimulation of Raf-1 phosphorylation with bFGF. Consistent with the effect on Raf-1 phosphorylation, dbcAMP and isoproterenol completely prevented bFGF-induced phosphorylation of MAP kinase kinases, target proteins of Raf-1. Our observations suggest that cAMP-induced suppression of cell growth in astrocytes is due to the inhibitory effect on activation of MAP kinase and its translocation to the nucleus and that the site of the cAMP action is located at Raf-1 or the upstream site of Raf-1.

Cyclosporin A stimulation of glucose-induced insulin secretion in MIN6 cells

Effects of the immunosuppressant cyclosporin A (CsA), a specific inhibitor of Ca2+/calmodulin-dependent protein phosphatase (PP2B), were examined with regard to the induction of insulin secretion from MIN6 cells, a glucose-responsive cell line derived from mouse insulinoma. CsA had no effect on basal insulin secretion from MIN6 cells, but did increase glucose-, tolbutamide-, and KCl-induced insulin secretion. Treatment of the cells with CsA resulted in a dose-dependent increase in insulin secretion, which was maximal at 3 microM. CsA inhibited PP2B activity in a dose-dependent manner, and the increase in insulin secretion correlated with the decrease in PP2B activity. In 32P-labeled cells, treatment with CsA for 30 min increased phosphorylation of synapsin I-like protein by 50 +/- 5.7%. As revealed by one-dimensional phosphopeptide mapping of 32P-labeled synapsin I-like protein, treatment with CsA for 30 min increased phosphorylation of site II of synapsin I-like protein by 59 +/- 8%, which is phosphorylated by calmodulin kinase II. Messenger RNAs, which hybridize with complementary DNAs of calcineurin A and B subunits from rat brain, were detected in MIN6 cells. Western blot analysis showed a 61-kDa band, which interacts with rat brain calcineurin A antibody. Similar increases in secretagogue-induced insulin secretion with CsA were observed for HIT-T15 cells. These results suggest that CsA stimulates glucose-induced insulin secretion by inhibiting the activity of PP2B, an event that may be involved in mechanisms governing glucose-induced insulin secretion via dephosphorylation of synapsin I-like protein in MIN6 cells.

Expression of angiotensin-converting enzyme in remaining viable myocytes of human ventricles after myocardial infarction

BACKGROUND

Local ACE in the heart may be important in the pathophysiological state after myocardial infarction (MI). It is unknown, however, whether ACE is expressed in myocytes of the human heart.

METHODS AND RESULTS

Using a newly generated polyclonal antibody to a synthetic peptide corresponding to part of the human endothelial ACE sequence, we examined the localization of ACE in left ventricles of patients (n = 10) with MI obtained at left ventricular aneurysmectomy or autopsy and in the hearts of control subjects at autopsy (n = 10). The avidinbiotinylated peroxidase complex method was used for the immunohistochemical staining for ACE. In the left ventricles, positively stained myocytes for ACE were found in 8 of the 10 patients with MI. ACE immunoreactivity was seen in the remaining viable myocytes located near the infarct scar of the aneurysmal left ventricle and in nonmyocytes such as fibroblasts, macrophages, vascular smooth muscle cells, and endothelial cells within the scarred tissue. On the other hand, no immunoreactivity for ACE was detected in the ventricular myocytes of all control hearts obtained at autopsy.

CONCLUSIONS

We observe immunohistochemical staining for ACE in the left ventricular myocytes of the region adjacent to the infarct scar and in nonmyocytes. These results indicate that ACE is markedly increased on the edge of the infarct scar and suggest that local ACE may be important in the ventricular remodeling after MI.

Regulation of CCAAT/enhancer-binding protein family members by stimulation of glutamate receptors in cultured rat cortical astrocytes

Regulation of mRNA levels, DNA binding activities, and phosphorylation of CCAAT/enhancer-binding protein (C/EBP) family members by stimulation of glutamate receptors were studied in cultured rat cortical astrocytes. Indirect immunofluorescence and immunoblot analyses with specific antibodies to C/EBP family members revealed that both C/EBPbeta and C/EBPdelta but not C/EBPalpha are expressed in the nuclei of astrocytes. After exposure to glutamate, C/EBPbeta mRNA levels increased within 10 min, reached the maximal level at about 1 h, and returned to the basal level within 6 h. In contrast, C/EBPdelta mRNA levels decreased by 6 h and were recovered within 12 h. These changes in mRNA levels were accompanied by an increase and a decrease in proteins for C/EBPbeta and C/EBPdelta, respectively. Elevation of C/EBPbeta mRNA levels by glutamate treatment required an increase in intracellular Ca2+ concentration and depended on activations of protein kinase C and calmodulin-dependent protein kinases. Gel mobility shift analysis using nuclear extracts from the glutamate-treated cells showed increases in C/EBP site binding activities 2 h after the exposure to glutamate. Moreover, glutamate stimulated phosphorylation of C/EBPbeta in 32P-labeled astrocytes in a Ca2+-dependent manner. These results suggest that glutamate regulates functions of C/EBP family members in brain astrocytes through changes in mRNA levels of C/EBPbeta and C/EBPdelta as well as through phosphorylation of C/EBPbeta.

Phosphorylation of chromogranin A and catecholamine secretion stimulated by elevation of intracellular Ca2+ in cultured bovine adrenal medullary cells

We have recently isolated a new endogenous substrate of 70 kDa for Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) from bovine adrenal medullary cells (Yanagihara, N., Toyohira, Y., Yamamoto, H., Ohta, Y., Tsutsui, M., Miyamoto, E., and Izumi, F. (1994) Mol. Pharmacol. 46, 423-430). Here we report the sequence analysis of the 70-kDa protein and examine its phosphorylation by various protein kinases in vitro and by depolarization of the cultured cells. Protein sequencing and immunoblotting revealed that the 70-kDa protein is chromogranin A (CgA) or a closely related protein. Partially purified CgA was phosphorylated by cyclic AMP-dependent protein kinase and protein kinase C as well as CaM kinase II. Tryptic phosphopeptide mapping patterns of CgA differed among these protein kinases. In 32P-labeled bovine adrenal medullary cells, 56 mM K+ increased the phosphorylation of CgA and catecholamine secretion in similar time- and concentration-dependent manners, both of which were inhibited by 20 mM MgSO4, an inhibitor of voltage-dependent Ca2+ channels. These findings suggest that CgA serves as a substrate for several multifunctional protein kinases and that the elevation of the intracellular Ca2+ stimulates the phosphorylation of CgA associated with catecholamine secretion in cultured adrenal medullary cells.

Sequential changes of tau-site-specific phosphorylation during development of paired helical filaments

It has been reported that many tau sites in neurofibrillary tangles (NFT) are abnormally phosphorylated. We investigated the phosphorylation of tau in the hippocampus of nondemented patients and Alzheimer's disease patients by immunostaining with five site-specific antibodies against phosphorylated tau. In the pretangle stage, tau in neuropil threads was phosphorylated at serines 199, 202 and 409, numbered according to the longest human tau isoform, whereas tau in some neuronal soma was phosphorylated at serines 199, 202, 409 and 422. Tau at the stage of NFT was phosphorylated at serine 396 and threonine 231 in addition to serines 199, 202, 409 and 422. In the advanced stage, tau in ghost tangles was phosphorylated mainly at serine 396. These results suggest that the phosphorylation of each site in tau differs among the maturing stages of neurofibrillary change and that abnormal phosphorylation of tau in the neuronal soma occurs at 199, 202, 409 and 422 earlier than at threonine 231 and serine 396.

The effects of 2,5-hexanedione and acrylamide on myosin heavy chain isoforms of slow and fast skeletal muscles of the rat

We examined the effects of 2,5-hexanedione (2,5-HD) and acrylamide (ACR) on the muscle fiber types and myosin heavy chain (MHC) isoform composition of the slow-twitch soleus and fast-twitch plantaris muscles of rats. We employed two differently designed experiments with respect to the dosage levels and treatment periods for developing clinical neuropathies. When male Wistar rats were subcutaneously injected with 4.5 mmol of 2,5-HD/kg or 0.4 mmol of ACR/kg, 5 days a week, they developed paralysis of the hindlimbs in 4 weeks (high-dosage experiment). When they were subcutaneously injected with 3.5 mmol of 2,5-HD/kg or 0.35 mmol of ACR/kg, 5 days a week, paralysis of the hindlimbs did not develop until 6 weeks (low-dosage experiment). We examined mainly the rats treated with the neurotoxicants for 4 and 8 weeks in the high-and low-dosage experiments, respectively. Significant decreases in the maximum motor conduction velocity of the sciatic nerves were observed in the hindlimbs of the rats in both experiments. The weights of the soleus and plantaris muscles were significantly reduced in the 2,5-HD-treated rats in both experiments, while in the ACR-treated rats, the weights of both muscles decreased only in the low-dosage experiment. We could not detect any changes in the fiber type composition of the muscles by any of the treatments. However, biochemical analysis revealed decreases in the values (percentage) of the relative amounts of fast-type MHC IIa and IIb isoforms to total MHC isoforms in the 2,5-HD-treated rats, but not in the ACR-treated rats in the high-dosage experiment. In contrast, significant differences in the relative amounts of MHC isoforms were not observed after administration of the low dosage of 2,5-HD. These results suggest that 2,5-HD preferentially disorders the muscle fibers which contain the MHC II isoform. These effects may occur only after the relatively acute intoxication of 2,5-HD at a high dosage.

Ca(2+)/calmodulin-dependent protein kinase II inhibitor KN-62 inhibits adrenal medullary chromaffin cell functions independent of its action on the kinase

KN-62, an inhibitor of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II), inhibited significantly catecholamine secretion and tyrosine hydroxylase activity stimulated by acetylcholine in cultured bovine adrenal medullary cells. KN-62, however, showed an additional inhibitory effect on acetylcholine-induced 45Ca2+ influx, which is essential for functional responses. Carbachol-stimulated 22Na+ influx, veratridine-induced 22Na+ influx, and 56 mM K(+)-evoked 45Ca2+ influx were also attenuated by KN-62. Inhibitions by KN-62 of these ion influxes were correlated closely with those of catecholamine secretion. KN-04, which is a structural analogue of KN-62 but does not inhibit CaM kinase II activity, elicited inhibitory effects on the three kinds of stimulant-evoked ion influxes with an inhibitory potency similar to KN-62. These results suggest that KN-62 inhibits catecholamine secretion and tyrosine hydroxylase activation due to mainly its ion channel blockade on the plasma membrane rather than the inhibition of CaM kinase II activity in the cells.

Immunohistochemical examination of phosphorylated tau in granulovacuolar degeneration granules

Granulovacuolar degeneration (GVD) and neurofibrillary tangles (NFT) are neuropathological features in Alzheimer's disease (AD). The molecular mechanism of GVD formation remains unknown. Recent immunohistochemical investigations suggested a potential link of NFT to GVD formation. Enzyme histochemical studies and electronmicroscopic findings suggested that GVD is formed through lysosomal autophagy of intraneuronal substances. We recently demonstrated that in non-demented cases NFT was phosphorylated at serines 199, 202 and 422 in paired helical filament (PHF)-tau more than in serine 396, while NFT in AD cases was similarly phosphorylated at these four sites in tau. In this study, we demonstrated immunohistochemically a similar phosphorylation state of tau in GVD granules to that in NFT in both non-demented cases and AD patients by using a mouse monoclonal anti-tau antibody and three phosphorylation site-specific antibodies for PHF-tau, indicating that GVD granules and NFT are composed of similar phosphorylated-tau. However, we could not detect PHF structures within any GVD using electronmicroscopy, indicating that PHF itself is not phagocytized by lysosomes during GVD formation. Therefore, the source of GVD granules might be phosphorylated pre-PHF-tau.

[Evaluation of the training of the anesthesiologist]

In order to evaluate both the knowledge and the degree of competence acquired by anesthesiology trainees, we constructed a checklist and began using it in January 1994. Twenty-one trainees participated in a 3-month program ending in February 1995. By means of the checklist, we attempted to evaluate the extent of their individual attainment. The total number of anesthesia administered was 50.9 +/- 7.8 cases. The total anesthesia time was 212.0 +/- 32.8 hours, and 76.2 % of the anesthesia methods applied involved either nitrous oxide-sevoflurane or nitrous oxide-isoflurane. Using a 5-level rating scale, the self-assessment by the supervising doctor tended to be one point higher than the assessment by the trainees. We plan to improve this checklist to improve the results of training.

CaM kinase II in long-term potentiation

The observation that autophosphorylation converts CaM kinase II from the Ca(2+)-dependent form to the Ca(2+)-independent form has led to speculation that the formation of the Ca(2+)-independent form of the enzyme could encode frequency of synaptic usage and serve as a molecular explanation of "memory". In cultured rat hippocampal neurons, glutamate elevated the Ca(2+)-independent activity of CaM kinase II through autophosphorylation, and this response was blocked by an NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). In addition, we confirmed that high, but not low frequency stimulation, applied to two groups of CA1 afferents in the rat hippocampus, resulted in LTP induction with concomitant long-lasting increases in Ca(2+)-independent and total activities of CaM kinase II. In experiments with 32P-labeled hippocampal slices, the LTP induction in the CA1 region was associated with increases in autophosphorylation of both alpha and beta subunits of CaM kinase II 1 h after LTP induction. Significant increases in phosphorylation of endogenous CaM kinase II substrates, synapsin I and microtubule-associated protein 2 (MAP2), which are originally located in presynaptic and postsynaptic regions, respectively, were also observed in the same slice. All these changes were prevented when high frequency stimulation was applied in the presence of AP5 or a calmodulin antagonist, calmidazolium. Furthermore, in vitro phosphorylation of the AMPA receptor by CaM kinase II was reported in the postsynaptic density and infusion of the constitutively active CaM kinase II into the hippocampal neurons enhanced kainate-induced response. These results support the idea that CaM kinase II contributes to the induction of hippocampal LTP in both postsynaptic and presynaptic regions through phosphorylation of target proteins such as the AMPA receptor, MAP2 and synapsin I.

Overexpression of Ca2+/calmodulin-dependent protein kinase II inhibits neurite outgrowth of PC12 cells

To investigate the physiological role of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) in neuronal differentiation, we transfected the cDNA of the alpha subunit of mouse CaM kinase II (CaM kinase II alpha) into PC12 cells and established clonal cell lines that constitutively express the transfected CaM kinase II alpha gene. The expression of CaM kinase II alpha was confirmed by northern blot and immunoblot analyses. Northern blot analysis showed that the gamma and delta subunits of CaM kinase II are mainly expressed in PC12 cells. Treatment of the cells with ionomycin activated CaM kinase II alpha through autophosphorylation and generation of the Ca2+/calmodulin-dependent form. It is interesting that the neurite outgrowth induced by dibutyryl cyclic AMP was inhibited in these cell lines in accordance with the activities of overexpressed CaM kinase II alpha. The activity of cyclic AMP-dependent protein kinase showed similar levels among these cell lines. These results suggest that CaM kinase II is involved in the modulation of the neurite outgrowth induced by activation of the cyclic AMP system.

A role of Ca2+/calmodulin-dependent protein kinase II in the induction of long-term potentiation in hippocampal CA1 area

Long-term potentiation (LTP) in the CA1 area of the hippocampus is considered to be a synaptic model for learning and memory. The induction of LTP is initiated by activation of the NMDA glutamate receptor in the postsynaptic membrane and a subsequent increase in Ca2+ -influx into the neurons following glutamate release. The action of Ca2+ has been proposed to be mediated by Ca2+ -dependent protein kinases. Recent studies indicate that, among the protein kinases, Ca2+/calmodulin-dependent protein kinase II is implicated in the induction of LTP in the hippocampus.

In situ dephosphorylation of tau by protein phosphatase 2A and 2B in fetal rat primary cultured neurons

Using antibodies recognizing the phosphorylation state of specific sites, phosphorylation states of tau were monitored in fetal rat primary cultured neurons. When cultured neurons were treated with okadaic acid (OA) or calyculin A (CalA) at concentrations sufficient to inhibit protein phosphatase 2A (PP2A), phosphorylation of Ser-199/Ser-202 (numbered according to the human tau 441) and Ser-235 increased. On the other hand, treatment with Ca2+ ionophore, A23187, induced dephosphorylation of Ser-199/Ser-202, Thr-205, Ser-396 and Ser-404, and this dephosphorylation was repressed by inhibitors of protein phosphatase 2B (PP2B), cyclosporin A and FK506. These results indicate that PP2A and PP2B are differentially involved in dephosphorylation of tau in neurons.

Dephosphorylation of fetal-tau and paired helical filaments-tau by protein phosphatases 1 and 2A and calcineurin

We have reported that many sites of tau in fetal brain (fetal-tau) as well as in paired helical filaments (PHF-tau) are phosphorylated. In the present study, we used site-specific antibodies and peptide mapping to examine protein phosphatases involved in dephosphorylation of fetal-tau and PHF-tau. Immunoblot analysis and electrophoretic mobility showed that protein phosphatases 1 and 2A and calcineurin could dephosphorylate fetal-tau and PHF-tau. Phosphoserines 199, 202, 396, and 413 and phosphothreonine 231, numbered according to the longest human tau isoform, were dephosphorylated, as shown by the immunoblot analysis. Phosphoserine 422 was dephosphorylated by protein phosphatase 2A and calcineurin, but not by protein phosphatase 1. Peptide mapping with Achromobacter lyticus protease 1 showed that phosphoserines 199, 202, 235, and 396 and phosphothreonine 231 were dephosphorylated by protein phosphatases. Fetal-tau was more rapidly dephosphorylated by protein phosphatase 2A and calcineurin than PHF-tau. Interestingly, PHF-tau which had not been solubilized with guanidine HCl was little dephosphorylated by protein phosphatases. Thus, PHF-tau in neurofibrillary tangles of Alzheimer's disease brain is likely to be resistant to dephosphorylation by protein phosphatases.

Modifications of neurofilament proteins by possible metabolites of allyl chloride in vitro

Chronic exposure to allyl chloride (ALL) is known to produce a central-peripheral distal axonopathy. In relation to the mechanism(s), the present study was conducted to examine the abilities of ALL and its putative metabolites, i.e., epichlorohydrin, glycerol alpha-monochlorohydrin, allyl alcohol and acrolein to cross-link proteins in vitro. Neurofilament-riched cytoskeletal proteins (1mg/ml) and ovalbumin (10mg/ml) were incubated with 160 mM tested chemicals except for acrolein at 0.5 mM and 1 mM. Time-dependent studies by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed that not only ALL, but also acrolein and epichlorohydrin exerted chemical modifications on axonal cytoskeletal proteins; while only acrolein-treated ovalbumin could manifest evidence of polymerization of the protein. Immunoblotting of PAGE-separated proteins confirmed that the high molecular weight proteins on the top of SDS-PAGEs were NF antigen-contained covalent cross-linked materials.

[The clinical scoring system for predicting the risk of intraoperative and postoperative bronchospasm]

Patients with bronchial asthma (BA) are usually considered to have a high risk of developing bronchospasm (BS) during anesthesia. Our clinical scoring system for preoperative assessment in BA patients was used to predict the risk of intraoperative and postoperative BS. Thirty two patients with a history of BA were studied retrospectively, assessing preoperatively with our clinical scoring system; Bronchial Asthma Risk Index Score (BARIS). BARIS was composed of a total of ten items. Five of them were scored before inhalation of bronchodilators and remaining five items were scored after the inhalation. Each item was scored by 0, 1, or 2. Four of 32 patients developed BS peroperatively (BS group). Twenty eight patients developed no BS peroperatively (control group). The total scores of pre-inhalation five items were 4.8 +/- 1.9 in BS group and 2.0 +/- 1.3 in control group, and the total scores of post-inhalation five items were 4.0 +/- 1.4 in BS group and 1.3 +/- 1.0 in control group. There were significant differences between the two groups (P < 0.05, Mann-Whitney U-test). We conclude that BARIS is useful in evaluating the risk of peroperative bronchospasm in patients with a history of bronchial asthma.

Activation of mitogen-activated protein kinase in cultured rat hippocampal neurons by stimulation of glutamate receptors

Mitogen-activated protein kinase (MAP kinase) was activated by stimulation of glutamate receptors in cultured rat hippocampal neurons. Ten micromolar glutamate maximally stimulated MAP kinase activity, which peaked during 10 min and decreased to the basal level within 30 min. Experiments using glutamate receptor agonists and antagonists revealed that glutamate stimulated MAP kinase through NMDA and metabotropic glutamate receptors but not through non-NMDA receptors. Glutamate and its receptor agonists had no apparent effect on MAP kinase activation in cultured cortical astrocytes. Addition of calphostin C, a protein kinase C (PKC) inhibitor, or down-regulation of PKC activity partly abolished the stimulatory effect by glutamate, but the MAP kinase activation by treatment with ionomycin, a Ca2+ ionophore, remained intact. Lavendustin A, a tryrosine kinase inhibitor, was without effect. In experiments with 32P-labeled hippocampal neurons, MAP kinase activation by glutamate was associated with phosphorylation of the tyrosine residue located on MAP kinase. However, phosphorylation of Raf-1, the c-raf protooncogene product, was not stimulated by treatment with glutamate. Our observations suggest that MAP kinase activation through glutamate receptors in hippocampal neurons is mediated by both the PKC-dependent and the Ca(2+)-dependent pathways and that the activation of Raf-1 is not involved.

Ca2+/calmodulin-dependent protein kinase II and synapsin I-like protein in mouse insulinoma MIN6 cells

Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) may play a key role in the regulation of insulin secretion. We obtained evidence for the presence of CaM kinase II and its substrate, a 84-kilodalton (kDa) protein, in mouse insulinoma MIN6 cells. CaM kinase II from MIN6 cells has one subunit of 55 kDa, determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, is autophosphorylated in a Ca2+/CaM-dependent manner, and phosphorylates several substrates that serve for rat brain CaM kinase II. In the membrane fraction of MIN6 cells, we identified a 84-kDa protein that was immunoreactive with the antirat brain synapsin I antibody. One-dimensional phosphopeptide mapping by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed the sites of the phosphorylation by cAMP-dependent protein kinase (cAMP kinase) and that by CaM kinase II to be site 1 (10 kDa) and site 2 (30 kDa), respectively, therefore, the same as for rat brain synapsin I. In this context, we tentatively termed it synapsin I-like protein. In 32P-labeled cells, nonfuel insulin secretagogues, such as ionomycin, KCl, and tolbutamide, and a fuel secretagogue, glucose, stimulated autophosphorylation of CaM kinase II and the phosphorylation of synapsin I-like protein. These secretagogues potentiated the Ca(2+)-independent activity of CaM kinase II and secretion of insulin from MIN6 cells. The 84-kDa protein is apparently a newly identified member of the synapsin family. We suggest that CaM kinase II regulates insulin secretion via phosphorylation of synapsin I-like protein.

Phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) by proline-directed protein kinases and its dephosphorylation

We identified two major substrates for the proline-directed protein kinases--cdc2 kinase and tau protein kinase II (TPKII)--in the cytosol fraction from rat brains. The molecular masses of the proteins were 80 and 46 kDa. Because the 80-kDa protein was phosphorylated by protein kinase C and was heat stable, we examined the possibility that the protein might be myristoylated alanine-rich C kinase substrate (MARCKS). On the basis of a comparison between the properties of the 80-kDa protein and purified MARCKS, we concluded that the 80-kDa protein is indeed MARCKS. The amounts of phosphate incorporated into MARCKS by protein kinase C, cdc2 kinase, and TPKII were 1.7, 1.4, and 0.6 mol/mol of the protein, respectively. Two-dimensional tryptic peptide mapping indicated that phosphorylation sites by protein kinase C and proline-directed protein kinases completely differed. Only the seryl residue was phosphorylated by protein kinase C, whereas both seryl and threonyl residues were phosphorylated by cdc2 kinase and TPKII. Phosphorylation of MARCKS by protein kinase C inhibited the binding to calmodulin, whereas phosphorylation by cdc2 kinase and TPKII significantly increased the binding to calmodulin. The holoenzyme of protein phosphatase 2A dephosphorylated MARCKS that had been phosphorylated by protein kinase C, cdc2 kinase, or TPKII, whereas calcineurin was unable to dephosphorylate it. These results suggest that cdc2 kinase and TPKII regulate the functions of MARCKS in different ways from protein kinase C.