Neurogranin (Ng) is a brain-specific, postsynaptically located protein kinase C (PKC) substrate, highly expressed in the cortex, hippocampus, striatum, and amygdala. This protein is a Ca2-sensitive calmodulin (CaM)-binding protein whose CaM-binding affinity is modulated by phosphorylation and oxidation. To investigate the role of Ng in neural function, a strain of Ng knockout mouse (KO) was generated. Previously we reported (Pak, J. H., Huang, F. L., Li, J., Balschun, D., Reymann, K. G., Chiang, C., Westphal, H., and Huang, K. -P. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 11232- 11237) that these KO mice displayed no obvious neuroanatomical abnormality, but exhibited deficits in learning and memory and activation of Ca2/CaM-dependent protein kinase II. In this report, we analyzed several downstream phosphorylation targets in phorbol 12-myristate 13-acetate-and forskolin-treated hippocampal slices from wild type (WT) and KO mice. Phorbol 12-myristate 13-acetate caused phosphorylation of Ng in WT mice and promoted the translocation of PKC from the cytosolic to the particulate fractions of both the WT and KO mice, albeit to a lesser extent in the latter. Phosphorylation of downstream targets, including mitogenactivated protein kinases, 90-kDa ribosomal S6 kinase, and the cAMP response element binding protein (CREB) was significantly attenuated in KO mice. Stimulation of hippocampal slices with forskolin also caused greater stimulation of protein kinase A (PKA) in the WT as compared with those of the KO mice. Again, phosphorylation of the downstream targets of PKA was attenuated in the KO mice. These results suggest that Ng plays a pivotal role in regulating both PKC-and PKA-mediated signaling pathways, and that the deficits in learning and memory of spatial tasks detected in the KO mice may be the result of defects in the signaling pathways leading to the phosphorylation of CREB.

Neurogranin (Ng) is a brain-specific, postsynaptically located protein kinase C (PKC) substrate, highly expressed in the cortex, hippocampus, striatum, and amygdala. This protein is a Ca2-sensitive calmodulin (CaM)-binding protein whose CaM-binding affinity is modulated by phosphorylation and oxidation. To investigate the role of Ng in neural function, a strain of Ng knockout mouse (KO) was generated. Previously we reported (Pak, J. H., Huang, F. L., Li, J., Balschun, D., Reymann, K. G., Chiang, C., Westphal, H., and Huang, K. -P. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 11232- 11237) that these KO mice displayed no obvious neuroanatomical abnormality, but exhibited deficits in learning and memory and activation of Ca2/CaM-dependent protein kinase II. In this report, we analyzed several downstream phosphorylation targets in phorbol 12-myristate 13-acetate-and forskolin-treated hippocampal slices from wild type (WT) and KO mice. Phorbol 12-myristate 13-acetate caused phosphorylation of Ng in WT mice and promoted the translocation of PKC from the cytosolic to the particulate fractions of both the WT and KO mice, albeit to a lesser extent in the latter. Phosphorylation of downstream targets, including mitogenactivated protein kinases, 90-kDa ribosomal S6 kinase, and the cAMP response element binding protein (CREB) was significantly attenuated in KO mice. Stimulation of hippocampal slices with forskolin also caused greater stimulation of protein kinase A (PKA) in the WT as compared with those of the KO mice. Again, phosphorylation of the downstream targets of PKA was attenuated in the KO mice. These results suggest that Ng plays a pivotal role in regulating both PKC-and PKA-mediated signaling pathways, and that the deficits in learning and memory of spatial tasks detected in the KO mice may be the result of defects in the signaling pathways leading to the phosphorylation of CREB.

Previous studies have shown that the level of total conventional protein kinase C (cPKC) membrane translocation (activation) was increased in brain of hypoxic preconditioned mice. In order to find out which isoform of cPKC may participate in the development of cerebral hypoxic preconditioning (HPC), we used Western bolt and immunohistochemistry to observe the effects of repetitive hypoxic exposure (H1-H6, n=6 for each group) on the level of cPKC isoform-specific protein expression and its membrane translocation in cortex and hippocampus of mice. We found that levels of cPKC βII and γ membrane translocation were increased significantly (p<0.05 versus normoxic H0 group, n=6) in response to repetitive hypoxic exposure (H1-H4) at early phase of hypoxic preconditioning, but no significant changes of cPKC α and βI membrane translocation while cPKC α, βI, βII and γ protein expression were found both in hippocampus and cortex. In addition, an extensive subcellular redistribution of cPKC βII and γ was detected by immunohistochemistry staining in cortex after repetitive hypoxic exposures (H3). However, a significant decrease in the expression of cPKC γ protein (p<0.05 versus H0 group) was found only in cortex of delayed hypoxic preconditioned mice (H5-6). These results suggested that the activation of cPKC βII and γ may be involved in the early phase of cerebral hypoxic preconditioning and the changes in cPKC γ protein expression may participate in the development of late phase of cerebral hypoxic preconditioning, as well as selective vulnerability to hypoxia both in cortex and hippocampus.

Neurogranin (NG) binding of calmodulin (CaM) at its IQ domain is sensitive to Ca2+ concentration and to modifications by protein kinase C (PKC) and oxidants. The PKC phosphorylation site of NG is within the IQ domain whereas the four oxidant-sensitive Cys residues are outside this region. These Cys residues were oxidized forming two pairs of intramolecular disulfides, and could also be glutathiolated by S-nitrosoglutathione resulting in the incorporation of four glutathiones per NG. Circular dichroism (CD) showed that modification of NG by phosphorylation, oxidation forming intramolecular disulfides, or glutathiolation did not affect the R-helical content of this protein. Mutation of the four Cys residues [Cys (-)-NG] to Gly and Ser did not affect the R-helical content either. Interaction of CaM with the reduced (red), glutathiolated (GS)-, or Cys(-)-NG in the Ca2+-free solution resulted in an increase in the R-helicity determined by their CD spectra, but relatively little change was seen with the oxidized NG (ox-NG) or phosphorylated NG (PO4-NG). The binding affinities between the various modified forms of NG and CaM were determined by CD spectrometry and sedimentation equilibrium: their affinities were Cys (-)-NG > red-NG, GS-NG > ox-NG > PO4-NG. Unlike Cys (-)-, red-, and GS-NG, neither ox-nor PO4-NG bound to a CaM-affinity column. Thus, both oxidation of NG to form intramolecular disulfides and phosphorylation of NG by PKC are effective in modulating the intracellular level of CaM. These results indicate that modification of NG to form intramolecular disulfides outside the IQ domain provides an alternative mechanism for regulation of its binding affinity to CaM.

S-Nitrosoglutathione (GSNO) undergoes spontaneous degradation that generates several nitrogen-containing compounds and oxidized glutathione derivatives. We identified glutathione sulfonic acid, glutathione disulfide S-oxide (GS (O) SG), glutathione disulfide S-dioxide, and GSSG as the major decomposition products of GSNO. Each of these compounds and GSNO were tested for their efficacies to modify rat brain neurogranin/RC3 (Ng) and neuromodulin/GAP-43 (Nm). Among them, GS (O) SG was found to be the most potent in causing glutathiolation of both proteins; four glutathiones were incorporated into the four Cys residues of Ng, and two were incorporated into the two Cys residues of Nm. Ng and Nm are two in vivo substrates of protein kinase C; their phosphorylations by protein kinase C attenuate the binding affinities of both proteins for calmodulin. When compared with their respective unmodified forms, the glutathiolated Ng was a poorer substrate and glutathiolated Nm a better substrate for protein kinase C. Glutathiolation of these two proteins caused no change in their binding affinities for calmodulin. Treatment of [35S] cysteine-labeled rat brain slices with xanthine/xanthine oxidase or a combination of xanthine/xanthine oxidase with sodium nitroprusside resulted in an increase in cellular level of GS (O) SG. These treatments, as well as those by other oxidants, all resulted in an increase in thiolation of proteins; among them, thiolation of Ng was positively identified by immunoprecipitation. These results show that GS (O) SG is one of the most potent glutathiolating agents generated upon oxidative stress.