Nuclear mechanisms of hypoxic cerebral injury in the newborn

Hypercapnia Causes Injury of the Cerebral Cortex and Cognitive Deficits in Newborn Piglets

In critically ill newborns, exposure to hypercapnia (HC) is common and often accepted in neonatal intensive care units to prevent severe lung injury. However, as a "safe" range of arterial partial pressure of carbon dioxide levels in neonates has not been established, the potential impact of HC on the neurodevelopmental outcomes in these newborns remains a matter of concern. Here, in a newborn Yorkshire piglet model of either sex, we show that acute exposure to HC induced persistent cortical neuronal injury, associated cognitive and learning deficits, and long-term suppression of cortical electroencephalogram frequencies. HC induced a transient energy failure in cortical neurons, a persistent dysregulation of calcium-dependent proapoptotic signaling in the cerebral cortex, and activation of the apoptotic cascade, leading to nuclear deoxyribonucleic acid fragmentation. While neither 1 h of HC nor the rapid normalization of HC was associated with changes in cortical bioenergetics, rapid resuscitation resulted in a delayed onset of synaptosomal membrane lipid peroxidation, suggesting a dissociation between energy failure and the occurrence of synaptosomal lipid peroxidation. Even short durations of HC triggered biochemical responses at the subcellular level of the cortical neurons resulting in altered cortical activity and impaired neurobehavior. The deleterious effects of HC on the developing brain should be carefully considered as crucial elements of clinical decisions in the neonatal intensive care unit.

Evaluation of neuronal apoptosis precursors in an experimental model of acute normovolemic hemodilution

Background

The effects of acute anemia on neuronal cells and the safe limits of hematocrit are not well established. The objective of this study was to evaluate neuronal pro- and anti-apoptotic Bax and Bcl-x proteins, caspase-3 and -9 activity, and DNA fragmentation after acute normovolemic hemodilution (ANH).

Methods

Twenty-four pigs were anesthetized and randomized into 4 groups: Sham, ANH to 15% hematocrit (ANH15%), ANH to 10% hematocrit (ANH10%) and hypoxia (Hx). ANH was achieved by simultaneous blood withdrawal and hydroxyethyl starch infusion. Hx consisted of ventilation with a 6% inspired oxygen fraction for 60 minutes. Bax and Bcl-x proteins as well as DNA fragmentation were evaluated in cortical nuclear and mitochondrial fractions. Caspase-3 and -9 activity was evaluated in the cortical mitochondrial and hippocampal cytosolic fractions. The data were compared using analysis of variance followed by Tukey’s test (P<0.05).

Results

No changes were observed in Bax protein expression after hemodilution in the ANH15% and ANH10% groups compared to the Sham group. Bax expression in the Hx group was increased in the nuclear and mitochondrial fractions compared to all other groups. No significant difference was observed in Bcl-x expression. Caspase-3 and -9 activity in the cytosolic and mitochondrial fractions was different in the Hx group compared to all other groups. No statistical significance in DNA fragmentation was found among the Sham, ANH15% or ANH10% groups.

Conclusion

ANH to 10 and 15% hematocrit did not induce alterations in apoptosis precursors, suggesting that cerebral oxygenation was preserved during these anemic states.

Phosphorylation of caspase-9 in the cytosolic fraction of the cerebral cortex of newborn piglets following hypoxia

We have previously shown that hypoxia leads to increased expression and increased activity of caspase-9 in the cerebral cortex of newborn piglets. Previous studies have demonstrated the importance of caspase-9 in the initiation of the apoptotic cascade, however, the mechanism of caspase-9 activation is not well understood. Experiments were conducted on newborn piglets 2-3 days of age that were anesthetized and mechanically ventilated. Hypoxia was induced by lowering the FiO(2) to 0.05-0.07 x 1h, and was confirmed biochemically by demonstrating decreased levels of ATP and PCr in the hypoxic groups in comparison with the normoxic group. The ATP level was 1.99+/-0.66 in the hypoxic group versus 4.10+/-0.19 in the normoxic group, P<0.05, and the PCr value was 0.68+/-0.14 in the hypoxic group, compared to 2.98+/-0.39 in the normoxic group, P<0.05. The cytosol of the neuronal nuclei from the cerebral cortex was probed with anti-phosphorylated Ser(196) caspase-9 antibody, using Western blot analysis. Protein bands were analyzed using image densitometry. In both the hypoxic and normoxic samples, protein bands were demonstrated just above the 50 kDa marker. Phosphorylated caspase-9 expression in OD x mm(2) was 43.85+/-8.4 in the normoxic group and 67.6+/-9.88 in the hypoxic group, P<0.05. The results of this study demonstrate that caspase-9, a key protein in hypoxia induced apoptosis, is phosphorylated at the Ser(196) site during hypoxia. The results demonstrate that hypoxia results in a post-translational modification of caspase-9 at Ser(196), which may alter the activity of caspase-9 in the hypoxic newborn brain.

Mechanism of caspase-3 activation during hypoxia in the cerebral cortex of newborn piglets

We have previously shown that the activity and the expression of caspase-9 and caspase-3 were increased during hypoxia in the cerebral cortex of newborn piglets. The present study was conducted to test the hypothesis that the hypoxia-induced activation of caspase-3 in the cerebral cortex of newborn piglets is mediated by caspase-9. Twenty-two newborn piglets were randomly assigned to four groups: normoxic (Nx), normoxic pretreated with a selective caspase-9 inhibitor, Z-Leu-Glu(OMe)-His-Asp(OMe)-Fluoromethyl ketone (Z-LEHD-FMK) (Nx+LEHD), hypoxic (Hx), and hypoxic pretreated with Z-LEHD-FMK (Hx+LEHD). Cerebral tissue hypoxia was confirmed biochemically by measuring ATP and phosphocreatine. Caspase-9 and -3 activities were determined spectrofluorometrically. The expression of caspase-9 and -3 proteins was measured by Western blot analysis using active enzyme specific antibodies. Cytosolic caspase-9 activity (nmol/mg protein/h) was 3.70+/-0.40 in Nx, 3.56+/-0.31 in Nx+LEHD (p=NS versus Nx), 4.99+/-0.64 in Hx (p<0.05 versus Nx), and 3.73+/-0.80 in Hx+LEHD (p<0.05 versus Hx, p=NS versus Nx). Cytosolic caspase-3 activity (nmol/mg protein/h) was 7.80+/-1.17 in Nx, 8.15+/-0.87 in Nx+LEHD (p=NS versus Nx), 13.07+/-0.78 in Hx (p<0.05 versus Nx), and 10.05+/-2.09 in Hx+LEHD (p<0.05 versus Hx) The density (ODxmm(2)) of active caspase-9 protein was 18.52+/-1.89 in Nx, 20.53+/-1.12 in Nx+LEHD (p=NS versus Nx), 32.36+/-5.03 in Hx (p<0.05 versus Nx), and 19.94+/-3.59 in Hx+LEHD (p<0.05 versus Hx, p=NS versus Nx). The density (ODxmm(2)) of active caspase-3 protein was 55.87+/-8.73 in Nx, 55.69+/-8.18 in Nx+LEHD (p=NS versus Nx), 94.10+/-12.05 in Hx (p<0.05 versus Nx), and 56.12+/-14.56 in Hx+LEHD (p<0.05 versus Hx, p=NS versus Nx). These data show that administration of a selective caspase-9 inhibitor, Z-LEHD-FMK, prior to hypoxia prevents the hypoxia-induced increase in caspase-3 activity and the expression of active caspase-3 protein. We conclude that the hypoxia-induced activation of caspase-3 during hypoxia in the cerebral cortex of newborn piglets is mediated by caspase-9.

Differential expression of apoptotic proteins following hypoxia-induced CREB phosphorylation in the cerebral cortex of newborn piglets

The present study investigates the correlation between the hypoxia-induced phosphorylation of cyclic AMP response element binding protein and the expression of apoptotic proteins (proapoptotic proteins Bax and Bad and antiapoptotic proteins Bcl-2 and Bcl-xl) during hypoxia in the cerebral cortex of newborn piglets. Piglets were divided into normoxic (Nx) and hypoxic (Hx, FiO(2)=0.06 for 1 h) groups. Cerebral tissue hypoxia was documented by ATP and phosphocreatine (PCr) levels. Ser(133) phosphorylation of cyclic AMP response element binding (CREB) protein was determined by Western blot analysis using a specific anti-phosphorylated Ser(133)-CREB protein antibody. The expression of apoptotic proteins was determined by using specific anti-Bax, anti-Bad, anti-Bcl-2 and anti-Bcl-xl antibodies. ATP and PCr values (mumoles/g brain) in Hx were significantly different from Nx (ATP: 4.40 +/- 0.39 in Nx vs. 1.19 +/- 0.44 in Hx, P<0.05 vs. Nx; PCr: 3.60 +/- 0.40 in Nx vs. 0.70 +/- 0.31 in Hx, P<0.05 vs. Nx). Ser(133) phosphorylated CREB protein (OD x mm(2)) was 74.55 +/- 4.75 in Nx and 127.13 +/- 19.36 in Hx (P<0.05 vs. Nx). The expression of proapoptotic proteins Bax and Bad increased and strongly correlated with the increase in CREB protein phosphorylation (correlation coefficient r=0.82 and r=0.85, respectively). The expression of antiapoptotic proteins Bcl-2 and Bcl-xl did not show correlation with CREB protein phosphorylation. We conclude that cerebral hypoxia results in differential regulation of CREB protein-mediated expression of proapoptotic and antiapoptotic proteins in the cerebral cortex of newborn piglets. We propose that the increased expression of proapoptotic vs antiapoptotic genes will lead to an increased potential for apoptotic programmed cell death in the Hx newborn brain.

Effect of resuscitation with 21% oxygen and 100% oxygen on NMDA receptor binding characteristics following asphyxia in newborn piglets

The present study investigated the effect of reventilation with 21% and 100% oxygen following asphyxia in newborn piglets on NMDA receptor binding characteristics, Na(+), K(+)-ATPase activity, and lipid peroxidation. After achieving a heart rate less than 60 beats per minute, asphyxiated piglets were reventilated with 21% oxygen or 100% oxygen. (3)[H]MK-801 binding showed the Bmax in the 21% and 100% groups to be 1.53 +/- 0.43 and 1.42 +/- 0.35 pmol/mg protein (p = ns). Values for Kd were 4.56 +/- 1.29 and 4.17 +/- 1.05 nM (p = ns). Na(+), K(+)-ATPase activity in the 21% and 100% groups were 23.5 +/- 0.9 and 24.4 +/- 3.9 micromol Pi/mg protein/h (p = ns). Conjugated dienes (0.05 +/- 0.02 vs. 0.07 +/- 0.03 micromol/g brain) and fluorescent compounds (0.54 +/- 0.05 vs. 0.78 +/- 0.19 microg quinine sulfate/g brain), were similar in both groups (p = ns). Though lipid peroxidation products trended higher in the 100% group, these data show that NMDA receptor binding and Na(+), K(+)-ATPase activity were similar following reventilation with 21% or 100% oxygen after a single episode of mild asphyxia.

Mechanism of Ca(2+)/calmodulin-dependent protein kinase IV activation and of cyclic AMP response element binding protein phosphorylation during hypoxia in the cerebral cortex of newborn piglets

Previously we showed that hypoxia results in increased neuronal nuclear Ca(2+) influx, Ca(2+)/calmodulin-dependent protein kinase IV activity (CaM KIV) and phosphorylation of c-AMP response element binding (CREB) protein. The aim of the present study was to understand the importance of neuronal nuclear Ca(2+) in the role of CaM KIV activation and CREB protein phosphorylation associated with hypoxia. To accomplish this the present study tests the hypothesis that clonidine administration will block increased nuclear Ca(2+) influx by inhibiting high affinity Ca(2+)/ATPase and prevent increased CaM KIV activity and CREB phosphorylation in the neuronal nuclei of the cerebral cortex of hypoxic newborn piglets. To accomplish this piglets were divided in three groups: normoxic, hypoxic, and hypoxic-treated with clonidine. The piglets that were in the Hx+Cl group received clonidine 5 min prior to hypoxia. Cerebral tissue hypoxia was confirmed biochemically by tissue levels of ATP and phosphocreatine (PCr). The data show that clonidine prevents hypoxia-induced increase in CaM KIV activity and CREB protein phosphorylation. We conclude that the mechanism of hypoxia-induced activation of CaM KIV and CREB phosphorylation is nuclear Ca(2+) influx mediated. We speculate that nuclear Ca(2+) influx is a key step that triggers CREB mediated transcription of apoptotic proteins and hypoxic mediated neuronal death.

Effects of severe hypocapnia on expression of bax and bcl-2 proteins, DNA fragmentation, and membrane peroxidation products in cerebral cortical mitochondria of newborn piglets

BACKGROUND

Hypocapnia occurs in the newborn infant inadvertently or as a therapeutic modality and may result in neuronal and mitochondrial alterations in the newborn brain. Since mitochondria regulate apoptosis, these alterations may initiate a cascade of reactions that lead to apoptotic cell death.

OBJECTIVES

This study tests the hypothesis that hypocapnia results in increased expression of the pro-apoptotic protein Bax, fragmentation of DNA and membrane lipid peroxidation in cerebral cortical mitochondria (mt) of newborn piglets.

METHODS

Studies were performed in three groups of anesthetized normoxic newborn piglets: hypocapnic (H, n = 5), ventilated at a PaCO(2) of 11-15 mm Hg; normocapnic (N, n = 5), ventilated at a PaCO(2) of 40 mm Hg; and corrected normocapnic (CN, n = 4), ventilated as H with CO(2) added to maintain normocapnia. Tissue ATP and phosphocreatine levels were determined. Mitochondrial membrane proteins were separated, transblotted and probed with antibodies to Bax and Bcl-2. Bands were detected by enhanced chemiluminescence and analyzed by imaging densitometry. mtDNA was isolated. Cell and mitochondrial membrane lipid peroxidation products were measured spectrofluorometrically.

RESULTS

ATP and PCr concentrations were similar in the 3 groups. The ratio of Bax/Bcl-2 increased significantly in H compared to N and CN. mtDNA fragmentation was also significantly greater in H compared to N or CN. Membrane lipid peroxidation was higher in H than in N or CN; and in CN compared to N.

CONCLUSIONS

The data demonstrate that severe hypocapnia results in increased Bax expression, DNA fragmentation, and membrane lipid peroxidation in mitochondria of cerebral cortical neurons of newborn piglets, and may result in apoptotic cell death.

Nuclear Ca(++)-influx, Ca (++)/calmodulin-dependent protein kinase IV activity and CREB protein phosphorylation during post-hypoxic reoxygenation in neuronal nuclei of newborn piglets: the role of nitric oxide

The present study tests the hypothesis that post-hypoxic reoxygenation results in an nitric oxide (NO)-mediated increase in nuclear Ca(++)-influx, increased calmodulin kinase (CaM kinase) IV activity, and increased Ser(133) phosphorylation of cyclic AMP response element binding (CREB) protein in neuronal nuclei of the cerebral cortex of newborn piglets. Piglets were divided into normoxic (Nx), hypoxic (Hx, FiO(2) = 0.07 for 1 h), hypoxic with 6 h reoxygenation (Hx + reox), and Hx + reox injected with 7-nitroindazole sodium salt (7-NINA), a nNOS inhibitor, immediately after hypoxia (Hx + 7-NINA). Cerebral tissue hypoxia was documented by ATP and phosphocreatine (PCr) levels. Nuclear Ca(++)-influx was determined using (45)Ca(++) and CaM kinase IV activity determined by (33)P-incorporation into syntide-2. Ser(133) phosphorylation of CREB protein was determined by Western blot analysis using a specific anti-phosphorylated Ser(133)-CREB protein antibody. ATP and PCr values in Hx, Hx + reox, and Hx + 7-NINA were significantly different from Nx (P < 0.05 versus Nx). Ca(++)-influx (pmoles/mg protein/min) was 3.79 +/- 0.91 in Nx; 11.81 +/- 2.54 in Hx (P < 0.05 versus Nx), 16.55 +/- 3.55 in Hx + reox (P < 0.05 versus Nx), and 12.40 +/- 2.93 in Hx + 7-NINA (P = NS versus Hx). CaM kinase IV activity (pmoles/mg protein/min) was 1,220 +/- 76 in Nx, 2,403 +/- 254 in Hx (P < 0.05 versus Nx), 1,971 +/- 147 in Hx + reox (P < 0.05 versus Hx), and 1,939 +/- 125 Hx + 7-NINA (P < 0.05 versus Hx). Ser(133) phosphorylated CREB protein expression (OD x mm(2)) was 87 +/- 2 in Nx, 203 +/- 24 in Hx (P < 0.05 versus Nx), 186 +/- 23 in Hx + reox (P < 0.05 Nx, P = NS versus Hx), and 128 +/- 10 in Hx + 7-NINA (P < 0.05 versus Hx and Hx + reox). The results show that post-Hx administration of 7-NINA prevents the increased nuclear Ca(++)-influx and CREB protein phosphorylation at Ser(133) during reox. We conclude that post-Hx increase in nuclear Ca(++)-influx leading to increased phosphorylation of CREB protein is mediated by NO derived from nNOS. However, hypoxia-induced increase in CaM Kinase IV activity decreased during the post-Hx reox. We propose that hypoxia-induced increase in CaM Kinase IV activity leads to increased phosphorylation of CREB protein and transcription of proapoptotic genes during post-Hx reox resulting in Hx neuronal death.

Increased neuronal nuclear calcium influx in neonatal seizures

We hypothesized that neonatal seizures lead to increased Ca(2+) influx (nCa(2+)I) in neuronal nuclei of newborn rats and that such increase is nitric-oxide mediated. Neuronal nuclear (45)Ca(2+) influx (nCa(2+)I) was measured in neuronal nuclei of 25 10-day-old male rat-pups newborn brains. They were divided into five groups (n = 5/group). (I) control; (II) hypoxia without seizures; (III) hypoxia with seizures; (IV) kainate, 2 mg/kg intraperitoneal (i.p.)-induced seizures and (V) 7-nitroindazole (7-NINA), 1 mg/kg i.p. pretreated, kainate-induced seizures. nCa(2+)I was significantly (P < 0.05) increased following hypoxia or seizures (hypoxic- or kainate-induced). Post-hypoxic seizures further enhanced nCa(2+)I increase induced by hypoxia (P < 0.05). 7-NINA abated the nCa(2+)I increase induced by kainate. We conclude that (1) kainate or hypoxia-induced seizures in newborn rats modify the neuronal nuclear membrane function, resulting in increased nCa(2+)I, (2) seizures exacerbate the hypoxia-induced increased nCa(2+)I incurred after hypoxia and (3) intranuclear calcium surges during kainate-induced neonatal seizures are nitric oxide-mediated.

Mechanisms of injury to the newborn brain

Preterm and ill term infants are at risk for brain injury and subsequent neurodevelopmental delay as a result of many perinatal factors. Outlined in this article are the basic science mechanisms by which hypoxia, hypocapnia, and hypercapnia may result in neuronal injury in the newborn brain.

Erythropoietin prevents long-term sensorimotor deficits and brain injury following neonatal hypoxia-ischemia in rats

Perinatal asphyxia accounts for behavioral dysfunctions that often manifest as sensorimotor, learning or memory disabilities throughout development and into maturity. Erythropoietin (Epo) has been shown to exert neuroprotective effects in different models of brain injury including experimental models of perinatal asphyxia. However, the effect of Epo on functional abilities following cerebral hypoxia-ischemia (HI) in neonatal rats is not known. The aim of the present study is to investigate the effect of Epo on sensorimotor deficits and brain injury induced by hypoxia-ischemia. Seven-day-old rats underwent unilateral, permanent carotid artery ligation followed by 1 h of hypoxia. Epo was administered as a single dose immediately after the hypoxic insult (2000 U/kg). The neuroprotective effect of Epo was evaluated at postnatal day 42 by using a battery of behavioral tests and histological analysis. The results of the present study suggest that Epo treatment immediately after HI insult significantly facilitated recovery of sensorimotor function. Consistently, histopathological evaluation demonstrated that Epo significantly attenuated brain injury and preserved the integrity of cerebral cortex. These findings indicate that long-term neuroprotective effect of Epo on neonatal HI-induced brain injury might be associated with the preservation of sensorimotor functions.