Changes in protein kinase C isozymes in the rat hippocampus following transient hypoxia

Identification of protein kinase C isoforms involved in cerebral hypoxic preconditioning of mice

Recently, accumulated studies have suggested that protein kinases C (PKC) play a central role in the development of ischemic-hypoxic preconditioning (I/HPC) in the brain. However, which types of PKC isoforms might be responsible for neuroprotection is still not clear, especially when the systematic investigation of PKC isoform-specific changes in brain regions was rare in animals with ischemic-hypoxic preconditioning. By using Western blot, we have demonstrated that the levels of cPKC betaII and gamma membrane translocation were increased in the early phase of cerebral hypoxic preconditioning. In this study, we combined the Western blot and immunostaining methods to investigate the effects of repetitive hypoxic exposure (H1-H4, n = 6 for each group) on membrane translocation and protein expression of several types of PKC isoforms, both in the cortex and hippocampus of mice. We found that the increased membrane translocation of nPKCepsilon (P < 0.05, versus normoxic H0) but not its protein expression levels in both the cortex and hippocampus during development of cerebral HPC in mice. However, there were no significant changes in both membrane translocation and protein expression levels of nPKCdelta, theta, eta, mu, and aPKC iota/lambda, zeta in these brain areas after hypoxic preconditioning. Similarly, an extensive subcellular redistribution of cPKCbetaII, gamma, and nPKCepsilon was observed by immunostaining in the cortex after three series of hypoxic exposures (H3). These results indicate that activation of cPKCbetaII, gamma, and nPKCepsilon might be involved in the development of cerebral hypoxic preconditioning of mice.

Reduced basal CAMKII levels in hippocampal CA1 region: Possible cause of stress-induced impairment of LTP in chronically stressed rats

Chronic psychosocial stress markedly reduces the expression of high-frequency stimulation (HFS)-evoked early long-term potentiation (LTP) in the CA1 region of the hippocampus of anesthetized rats. Immunoblotting was performed to determine changes in molecular levels of key signaling proteins that might be responsible for this inhibitory effect. Western blot analysis of the CA1 region demonstrates that chronic psychosocial stress decreases basal levels of calcium calmodulin kinase II (CaMKII), phosphorylated (P)-CaMKII, calmodulin, and protein kinase C (PKCgamma) while markedly increasing protein phosphatase 2B (calcineurin) levels. The decrease of basal levels of P-CaMKII may be triggered primarily by excessive dephosphorylation resulting from enhanced basal levels of calcineurin. The decline in the basal levels of the upstream molecules, PKCgamma and calmodulin may be a consequence of the diminished basal P-CaMKII levels. Analysis of signaling molecules in CA1 region of chronically stressed rat subjected to HFS in vivo showed only one difference compared to similarly stimulated control rats; no increase in P-CaMKII levels. Our results suggest that decreased P-CaMKII levels may be primarily responsible for the stress-induced reduction in LTP expression.

Partial HIF-1alpha deficiency impairs pulmonary arterial myocyte electrophysiological responses to hypoxia

Chronic hypoxia depolarizes and reduces K+ current in pulmonary arterial smooth muscle cells (PASMCs). Our laboratory previously demonstrated that hypoxia-inducible factor-1 (HIF-1) contributed to the development of hypoxic pulmonary hypertension. In this study, electrophysiological parameters were measured in PASMCs isolated from intrapulmonary arteries of mice with one null allele at the Hif1a locus encoding HIF-1alpha [Hif1a(+/-)] and from their wild-type [Hif1a(+/+)] littermates after 3 wk in air or 10% O2. Hematocrit and right ventricular wall and left ventricle plus septum weights were measured. Capacitance, K+ current, and membrane potential were measured with whole cell patch clamp. Similar to our laboratory's previous results, hypoxia-induced right ventricular hypertrophy and polycythemia were blunted in Hif1a(+/-) mice. Hypoxia increased PASMC capacitance in Hif1a(+/+) mice but not in Hif1a(+/-) mice. Chronic hypoxia depolarized and reduced K+ current density in PASMCs from Hif1a(+/+) mice. In PASMCs from hypoxic Hif1a(+/-) mice, no reduction in K+ current density was observed, and depolarization was significantly blunted. Thus partial deficiency of HIF-1alpha is sufficient to impair hypoxia-induced depolarization, reduction of K+ current density, and PASMC hypertrophy.

Differential alteration of catecholamine release during chemical hypoxia is correlated with cell toxicity and is blocked by protein kinase C inhibitors in PC12 cells

Release of neurotransmitters, including dopamine and glutamate, has been implicated in hypoxia/ischemia-induced alterations in neuronal function and in subsequent tissue damage. Although extensive studies have been done on the mechanism underlying the changes in glutamate release, few have examined the mechanism that is responsible for the changes in catecholamines. Rat pheochromocytoma-12 (PC12) cells synthesize, store, and release catecholamines including DA and NE. Therefore, we used HPLC and ED to evaluate extracellular DA and NE concentrations in a medium during chemical hypoxia in PC12 cells. Chemical hypoxia produced by KCN induced differential release of DA and NE. Under normal glucose conditions, KCN induced release of NE, but not DA. Under glucose-free conditions, KCN-induced release of DA was elevated transiently, whereas the release of NE increased progressively. Under parallel conditions, KCN biphasically elevated the level of cytosolic free calcium ([CA(2+)](i)) in glucose-free DMEM, peaking at 95 +/- 18 nM at 1,107 +/- 151 s, followed by a new plateau level at 249 +/- 24 nM sustained from 4,243 +/- 466 to 5,263 +/- 440 s. Cell toxicity, as measured by LDH release, was increased significantly by KCN in glucose-free DMEM but was diminished in the presence of glucose, and was correlated with DA release by chemical hypoxia. The protein kinase C (PKC) inhibitor GO6976 or staurosporine inhibited KCN-induced LDH release as well as the release of NE and DA. Taken together, selective activation of DA but not NE was correlated with the LDH release by chemical hypoxia, and was diminished with GO6976 or staurosporine. These results suggest that selective activation of PKC isoforms is involved in the chemical hypoxia-induced DA release, which may lead to neuronal cell toxicity.

Chronic hypoxia alters effects of endothelin and angiotensin on K+ currents in pulmonary arterial myocytes

We tested the hypothesis that chronic hypoxia alters the regulation of K+ channels in intrapulmonary arterial smooth muscle cells (PASMCs). Charybdotoxin-insensitive, 4-aminopyridine-sensitive voltage-gated K+ (K(V,CI)) and Ca2+-activated K+ (KCa) currents were measured in freshly isolated PASMCs from rats exposed to 21 or 10% O2 for 17-21 days. In chronically hypoxic PASMCs, K(V, CI) current was reduced and KCa current was enhanced. 4-Aminopyridine (10 mM) depolarized both normoxic and chronically hypoxic PASMCs, whereas charybdotoxin (100 nM) had no effect in either group. The inhibitory effect of endothelin (ET)-1 (10(-7) M) on K(V,CI) current was significantly reduced in PASMCs from chronically hypoxic rats, whereas inhibition by angiotensin (ANG) II (10(-7) M) was enhanced. Neither ET-1 nor ANG II altered K(Ca) current in normoxic PASMCs; however, both stimulated K(Ca) current at positive potentials in chronically hypoxic PASMCs. These results suggest that although modulation of K(V,CI) and KCa channels by ET-1 and ANG II is altered by chronic hypoxia, the role of these channels in the regulation of resting membrane potential was not changed.

Selective subcellular redistributions of protein kinase C isoforms by chemical hypoxia

The mechanisms of neuronal degeneration following hypoxia/ischemia remain undefined, but the processes include increases in neurotransmitter release, elevation of cytosolic-free calcium concentration, and changes in signal transduction pathways. Activation of the multigene family of protein kinase C (PKC) has been associated with the release of neurotransmitter and the survival of neurons. Therefore, to understand which PKC isozymes are involved in hypoxia/ischemia-induced neuronal degeneration, we examined PKC isozymes after chemical hypoxia (i.e., KCN exposure) in PC12 cells. Cell toxicity, as measured by lactate dehydrogenase (LDH) release, was increased significantly by KCN in glucose-free DMEM and was exaggerated by acute 12-O-tetradecanoyl phorbol-13-acetate (TPA) pretreatment. Under parallel conditions, KCN elevated cytosolic-free calcium ([Ca2+]i) in glucose-free but not in glucose containing DMEM, and TPA pretreatment did not exaggerate KCN's effect on [Ca2+]i. Thus, increases in [Ca2+]i are not sufficient for the synergistic toxic effect of KCN and TPA. In the glucose-free DMEM, selective PKC isozyme inhibitor Go 6976 at 10 nM completely inhibited KCN-induced LDH release and at higher concentrations (1 microM) inhibited the basal levels of LDH release. The protein levels of PKCs in the nuclear, membrane, and cytosolic fractions were measured by Western blot analysis using antibodies against specific isoforms. Two Ca2+-dependent (-alpha, -gamma) and four Ca2+-independent (-delta, -epsilon, -zeta, and -lambda) isozymes were identified and two isozymes (-beta and -theta) were not detected in the subcellular fractions of PC12 cells. Treatment of the cells with TPA significantly activated translocation of conventional PKC-gamma from the cytosol to the membrane and nuclear fractions and other PKC isozymes (-alpha, -delta, and -epsilon) from the cytosol to the membrane, but not atypical PKC-zeta and -lambda. Although only the levels in the nuclear PKC-gamma but not other PKC isozymes were increased significantly following KCN, the levels of cPKC-alpha and -gamma in the membrane mainly- and those and PKC-epsilon in the nucleus-were increased when KCN was combined with TPA. In addition, this condition (TPA + KCN) did not affect the TPA insensitive atypical isozymes, PKC-zeta or -lambda. Taking the results together, differential activation/translocation of PKC isozymes by KCN and TPA is important in the regulation of chemical hypoxia-induced cell injury in PC12 cells.

Delayed neuronal migration of protein kinase Cgamma immunoreactive cells in hippocampal CA1 area after 48 h of moderate hypoxemia in the near term ovine fetus

The brain is uniquely sensitive to disturbances in energy and oxygen supply, particularly during the early stage of life. Since hypoxemia can indirectly activate the intracellular messenger protein kinase C (PKC), we studied the PKCgamma-immunoreaction in the fetal hippocampal CA1 region of naive (n=4), instrumented control (n=7), and instrumented hypoxemic fetuses (n=14), at a mean gestational age of 127 days. Forty-eight hours of mild to moderate hypoxemia, were followed by a 48-h recovery period. Hypoxemia resulted in an increase in carotid blood flow (137% of control), and a shift towards a higher percentage of high-voltage electrocortical activity. After recovery, the fetal brain was fixated by perfusion of both carotid arteries, sectioned and immunostained for PKCgamma. The distribution of PKCgamma-immunoreactive cells was significantly changed after 48 h of hypoxemia in that the migration of cells (from the ventricular region towards the stratum pyramidale) was delayed (p<0.01) compared to naive and instrumented control animals. In contrast to the distribution, the relative total optical density of PKCgamma-ir cells and fibres in the CA1 hippocampal area was not significant different between the animal groups. We conclude that hypoxemia delayed migration of PKCgamma-ir cells, without neuronal degeneration.

Selective activation by bryostatin-1 demonstrates unique roles for PKC epsilon in neurite extension and tau phosphorylation

Phorbol esters such as 12-O-tetradeonyl phorbol-13 acetate (TPA) induce a time-dependent biphasic effect on protein kinase C (PKC)-mediated events by fostering translocation of cytosolic (latent) PKC to the plasma membrane (where it is activated). Continued treatment, however, depletes the cell's entire PKC complement and induces a functional stake of PKC inhibition. Previous studies from several laboratories have demonstrated that long-term TPA treatment, like treatment with PKC inhibitors, induces neuronal differentiation. Bryostatin-1 also induces translocation and overall downregulation of PKC following long-term treatment, yet, unlike TPA or PKC inhibitors, does not induce neuronal differentiation, promoting controversy regarding the role of PKC inhibition in neuronal differentiation. We demonstrate herein that, despite overall downregulation in human neuroblastoma cells, membrane-associated levels of one PKC isoform (PKC epsilon) are actually increased following long-term bryostatin-1 treatment. Since previous studies have implicated this PKC isoform in phosphorylation of the microtubule-associated protein tau and in neuritogenesis, we examined the consequences of long-term bryostatin treatment on these phenomena. Treatment with 25 n-100 M bryostatin-1 for 72 h increased tau phosphorylation and inhibited neuritogenesis. By contrast, treatment with either TPA or the PKC inhibitor staurosporine did not induce tau phosphorylation and induced neurite elaboration. Bryostatin-1 antagonized neurite induction by staurosporine. These findings provide additional evidence for a unique role of PKC epsilon in the regulation of tau phosphorylation and neuronal differentiation, and demonstrate that bryostatin-1 can function under certain conditions as a selective PKC epsilon activator even following long-term treatment.

Intracortical glutamate perfusion in vivo induces cellular alterations in specific protein kinase C isoforms and amyloid precursor protein

This study investigated the immunostaining of protein kinase C (PKC) isoforms and amyloid precursor protein (APP) in rat brain cortex and determined alterations following an excitotoxic challenge in vivo. Cellular alterations in APP and PKC isoforms (alpha, beta, gamma, delta, epsilon, and zeta) following glutamate perfusion in the rat parietal cortex were compared with NaCl perfusion. In all animals, two histological zones could be defined consistently, a necrotic core and a boundary zone immediately adjacent to the core. Following glutamate and NaCl perfusion, cellular immunoreactivity to PKC isoforms and amino-terminal APP was significantly reduced within the necrotic core. Striking carboxy-terminal APP immunoreactivity was observed in some neurons remaining within the necrotic core. In the boundary of the glutamate lesion, the perikarya of most neurons were intensely immunoreactive to PKC alpha and beta. Furthermore, within the boundary zone, enhanced immunoreactivity within neuronal perikarya was observed to amino-terminal APP and, to a lesser extent, carboxy-terminal APP. Increased immunostaining of PKC alpha and beta and APP at the boundary zone was a consistent feature of intracortical glutamate perfusion and was not observed following NaCl perfusion. There were minimal alterations in PKC isoforms gamma, delta, epsilon, and zeta, in the boundary region following intracortical glutamate or NaCl perfusion. There was no astrocytic response, as detected by GFAP immunoreactivity, at the boundary zone. These findings indicate that there is a topographical relationship between cellular alterations of specific PKC isoforms and APP following an excitotoxic challenge in vivo.