Inositol 1,4,5-triphosphate receptors and NAD(P)H mediate Ca2+ signaling required for hypoxic preconditioning of hippocampal neurons

Exposure of neurons to a non-lethal hypoxic stress greatly reduces cell death during subsequent severe ischemia (hypoxic preconditioning, HPC). In organotypic cultures of rat hippocampus, we demonstrate that HPC requires inositol triphosphate (IP3) receptor-dependent Ca2+ release from the endoplasmic reticulum (ER) triggered by increased cytosolic NAD(P)H. Ca2+ chelation with intracellular BAPTA, ER Ca2+ store depletion with thapsigargin, IP3 receptor block with xestospongin, and RNA interference against subtype 1 of the IP3 receptor all blunted the moderate increases in [Ca2+](i) (50-100 nM) required for tolerance induction. Increases in [Ca2+](i) during HPC and neuroprotection following HPC were not prevented with NMDA receptor block or by removing Ca2+ from the bathing medium. Increased NAD(P)H fluorescence in CA1 neurons during hypoxia and demonstration that NADH manipulation increases [Ca2+](i) in an IP3R-dependent manner revealed a primary role of cellular redox state in liberation of Ca2+ from the ER. Blockade of IP3Rs and intracellular Ca2+ chelation prevented phosphorylation of known HPC signaling targets, including MAPK p42/44 (ERK), protein kinase B (Akt) and CREB. We conclude that the endoplasmic reticulum, acting via redox/NADH-dependent intracellular Ca2+ store release, is an important mediator of the neuroprotective response to hypoxic stress.

Hypoxia increases calcium flux through cortical neuron glutamate receptors via protein kinase C

The effects of 30 s to 10 min hypoxia (PO2-10 mmHg) on glutamate receptor activity were studied in murine cortical neurons. Receptor activity was assessed as a rise in intracellular calcium concentration ([Ca2+]i) following a 10 s application of 1 mm glutamate or 100 micro mN-methy-d-aspartate (NMDA) in the presence of 0.1 mm Mg2+ and 10 micro m glycine. Change in [Ca2+]i elicited by glutamate increased 26% (n = 192, p < 0.001) and that to NMDA by 74% (n = 9, p < 0.01) during a 100-s period of hypoxia. After 10 min hypoxia, responses to glutamate were 62% smaller than those in normoxia, with increased basal intracellular [Ca2+]i predicting reduced receptor activity. When neurons were exposed to NMDA after 10 min of hypoxia, [Ca2+]i increases were 12% smaller than after 100 s hypoxia, but still 53% larger than in oxygenated neurons (n = 9, p = 0.01). Neurons expressed relatively similar amounts of NR2A, -B, -C, and -D subunits. The phosphorylation of NMDA NR1 subunits increased during hypoxia. Pre-treatment of neurons with a protein kinase C (PKC) inhibitor (chelerythrine, 10 micro m) prevented increases in N-methy-d-aspartate receptor (NMDAR) activity during hypoxia and reduced the phosphorylation of NR1 subunits. These results suggest that enhancement of glutamate receptor activity during the first minutes of hypoxia is mediated by phosphorylation of NMDARs by PKC and that other mechanisms, possibly involving intracellular calcium, limit glutamate receptor-mediated calcium influx during longer periods of hypoxia.

Moderate increases in intracellular calcium activate neuroprotective signals in hippocampal neurons

Although large increases in neuronal intracellular calcium concentrations ([Ca(2+)](i)) are lethal, moderate increases in [Ca(2+)](i) of 50-200 nM may induce immediate or long-term tolerance of ischemia or other stresses. In neurons in rat hippocampal slice cultures, we determined the relationship between [Ca(2+)](i), cell death, and Ca(2+)-dependent neuroprotective signals before and after a 45 min period of oxygen and glucose deprivation (OGD). Thirty minutes before OGD, [Ca(2+)](i) was increased in CA1 neurons by 40-200 nM with 1 nM-1 microM of a Ca(2+)-selective ionophore (calcimycin or ionomycin-"Ca(2+) preconditioning"). Ca(2+) preconditioning greatly reduced cell death in CA1, CA3 and dentate during the following 7 days, even though [Ca(2+)](i) was similar (approximately 2 microM) in preconditioned and control neurons 1 h after the OGD. When pre-OGD [Ca(2+)](i) was lowered to 25 nM (10 nM ionophore in Ca(2+)-free medium) or increased to 8 microM (10 microM ionophore), more than 90% of neurons died. Increased levels of the anti-apoptotic protein protein kinase B (Akt) and the MAP kinase ERK (p42/44) were present in preconditioned slices after OGD. Reducing Ca(2+) influx, inhibiting calmodulin, and preventing Akt or MAP kinase p42/44 upregulation prevented Ca(2+) preconditioning, supporting a specific role for Ca(2+) in the neuroprotective process. Further, in continuously oxygenated cultured hippocampal/cortical neurons, preconditioning for 30 min with 10 nM ionomycin reduced cell death following a 4 microM increase in [Ca(2+)](i) elicited by 1 microM ionomycin. Thus, a zone of moderately increased [Ca(2+)](i) before a potentially lethal insult promotes cell survival, uncoupling subsequent large increases in [Ca(2+)](i) from initiating cell death processes.

Oxygen sensitivity of NMDA receptors: relationship to NR2 subunit composition and hypoxia tolerance of neonatal neurons

Neonatal rats survive and avoid brain injury during periods of anoxia 25 times longer than adults. We hypothesized that oxygen activates and hypoxia suppresses NMDA receptor (NMDAR) responses in neonatal rat neurons, explaining the innate hypoxia tolerance of these cells. In CA1 neurons isolated from neonatal rat hippocampus (mean postnatal age [P] 5.8 days), hypoxia (PO(2) 10 mm Hg) reduced NMDA receptor-channel open-time percentage and NMDA-induced increase in [Ca(2+)](i) (NMDA DeltaCa(2+)) by 38 and 68% (P<0.01), respectively. In P20 neurons the reductions were not significant. In P3-10 CA1 neurons within intact hippocampal slices, hypoxia reduced NMDA DeltaCa(2+) by 52% (P=0.002) and decreased NMDA-induced death by 45% (P=0.004). Phalloidin, a microtubule stabilizer, prevented hypoxia-induced inhibition of NMDA DeltaCa(2+) in P3-10 neurons. To test whether NMDARs prevalent in neonates (NR1 plus NR2B or NR2D subunits) are inhibited by hypoxia compared with those in mature neurons (NR2A and NR2C), we expressed these receptors in Xenopus oocytes. Compared with responses in 21% O(2), hypoxia (PO(2) 17 mm Hg) reduced currents from neonatal type NR1/NR2D receptors by 25%, increased currents from NR1/NR2C by 18%, and had no effect on NR1/NR2A or NR1/NR2B. Modulation of NMDARs by hypoxia may play an important role in the hypoxia tolerance of the mammalian neonate. In addition, oxygen sensing by NMDARs could play a significant role in postnatal brain development.

Activation of the neuroprotective ERK signaling pathway by fructose-1,6-bisphosphate during hypoxia involves intracellular Ca2+ and phospholipase C

The mechanism of the neuroprotective action of the glycolytic pathway intermediate fructose-1,6-bisphosphate (FBP) may involve activation of a phospholipase-C (PLC) dependent MAP kinase signaling pathway. In this study, we determined whether FBP's capacity to decrease delayed cell death in hippocampal slice cultures is dependent on PLC signaling or activation of the intracellular Ca(2+)-MEK/ERK neuroprotective signaling cascade. FBP (3.5 mM) reduced delayed death from oxygen/glucose deprivation in CA1, CA3 and dentate neurons in slice cultures. The phospholipase-C inhibitor U73122 and the MEK1/2 inhibitor U0126 prevented this protection. In hippocampal and cortical neurons, FBP increased phospho-ERK1/2 (p42/44) immunostaining during hypoxic, but not normoxic conditions. Increased phospho-ERK immunostaining was dependent on PLC and also on MEK 1/2, an upstream regulator of ERK. Further, we found that FBP enhancement of phospho-ERK immunostaining depended on [Ca(2+)](i): PLC inhibition and the IP(3) receptor blocker xestospongin C prevented FBP from increasing [Ca(2+)](i) and increasing phospho-ERK levels. However, while FBP-induced increases in [Ca(2+)](i) were blocked by xestospongin and a PLC inhibitor, [Ca(2+)](i) increases induced by the neuroprotective growth factor BDNF were not prevented. We conclude that during hypoxia FBP initiates a series of neuroprotective signals which include PLC activation, small increases in [Ca(2+)](i), and increased activity of the MEK/ERK signaling pathway.

Neuroprotection and intracellular Ca2+ modulation with fructose-1,6-bisphosphate during in vitro hypoxia-ischemia involves phospholipase C-dependent signaling

The neuroprotectant fructose-1,6-bisphosphate (FBP) preserves cellular [ATP] and prevents catastrophic increases in [Ca2+]i during hypoxia. Because FBP does not enter neurons or glia, the mechanism of protection is not clear. In this study, we show that FBP's capacity to protect neurons and stabilize [Ca2+]i during hypoxia derives from signaling by a phospholipase-C-intracellular Ca2+-protein kinases pathway, rather than Ca2+ chelation or glutamate receptor inhibition. FBP reduced [Ca2+]i changes in hypoxic hippocampal neurons, regardless of [Ca2+]e, and preserved cellular integrity as measured by trypan blue or propidium iodide exclusion and [ATP]. FBP also prevented hypoxia-induced increases in [Ca2+]i when glucose was absent and when [Ca2+]e was increased to negate Ca2+ chelation by FBP. These protective effects were observed equally in postnatal day 2 (P2) and P16 neurons. Inhibiting glycolysis with iodoacetate eliminated the protective effects of FBP in P16 neurons. FBP did not alter Ca2+ influx stimulated by brief applications of NMDA or glutamate during normoxia or hypoxia, but did reduce the increase in [Ca2+]i produced by 10 min of glutamate exposure during hypoxia. Because FBP increases basal [Ca2+]i and stimulates membrane lipid hydrolysis, we tested whether FBP's protective action was dependent on phospholipase C signaling. The phospholipase C inhibitor U73122 prevented FBP-induced increases in [Ca2+]i and eliminated FBP's ability to stabilize [Ca2+]i and increase survival during anoxia. Similarly, FBP's protection was eliminated in the presence of the mitogen/extracellular signal protein kinase (MEK) inhibitor U0126. We conclude that FBP may produce neuroprotection via activation of neuroprotective signaling pathways that modulate Ca2+ homeostasis.

Differing effects of copper,zinc superoxide dismutase overexpression on neurotoxicity elicited by nitric oxide, reactive oxygen species, and excitotoxins

Overexpression of Cu,Zn superoxide dismutase (SOD1) reduces ischemic injury in some stroke models but exacerbates injury in a neonatal stroke model and in other settings. The current study used a SOD1 transgenic (SOD1-Tg) murine cortical culture system, derived from the same mouse strain previously used for the stroke models, to identify conditions that determine whether SOD1 overexpression in neurons is protective or detrimental. The nitric oxide (NO) donors S-nitroso-N-acetylpenicillamine, spermine-NONOate, and diethylamine-NONOate produced less death in SOD1-Tg neurons than in wild-type neurons (p < 0.01). Also, NO produced markedly less 3-nitrotyosine in SOD1-Tg cells. In contrast, the superoxide generator menadione produced significantly greater death and nearly twice as much 2'7'-dichlorofluorescein fluorescence in SOD1-Tg neurons than in wild-type neurons, suggesting increased peroxide formation in the SOD1-Tg cells. No significant difference was observed in the vulnerability of the two cell types to H2O2, the product of the SOD reaction. Overexpression of SOD1 also had no effect on neuronal vulnerability to glutamate, N-methyl-D-aspartate, or kainate. These observations suggest that SOD1 overexpression can reduce neuronal death under conditions where peroxynitrite formation is a significant factor, but may exacerbate neuronal death under conditions of rapid intracellular superoxide formation or impaired H2O2 disposal.

Colchicine-induced sprouting of the neuromuscular junction in the pigeon extensor digitorum longus muscle

Colchicine-induced motor endplate sprouting in the extensor digitorum longus muscle of the pigeon was examined. Ten days after the drug application sprouting from the endplate arborizations and nodes of Ranvier were observed. No concomitant changes in endplate surface area or in the degree of terminal branching could be demonstrated. Similarities between the sprouting patterns of the pigeon endplate and the mammalian endplate are discussed.

Colchicine-induced differential sprouting of the endplates on fast and slow muscle fibers in rat extensor digitorum longus, soleus and tibialis anterior muscles

The patterns of sprouting of motor endplates were examined in fast extensor digitorum longus and slow soleus muscles and in tibialis anterior muscles containing fast and slow muscle fiber types. A histochemical technique combining nerve silver impregnation and endplate cholinesterase staining was developed for this task. Temporal examination of the innervation was conducted 3, 7 and 10 days after either a 45 or 90 min application of the ipsilateral sciatic nerve with 5 mM colchicine. This dosage of drug did not cause detectable axon or muscle fiber degeneration, unlike 60 mM which was highly neurotoxic. At 3 days following treatment with the lower concentration, there were no significant differences in the percentages of intranodal, preterminal and ultraterminal sprouts between the normal (non-treated), sham-treated, contralateral systemic-control and drug-treated groups of muscles. By 7 and 10 days, the muscles on the drug-treated side exhibited significant increases in the 3 types of sprouts. Collateral sprouting was uncommon: most outgrowths remained on the muscle fibers innervated by the parent axons. Endplates in the tibialis anterior muscles of the control and drug-treated groups were classified Complex, Intermediate or Simple according to the relative degrees of branching of the terminal arbors. The occurrence of endplate classes and muscle fiber types was correlated in the superficial and deep regions of this muscle. Complex endplates innervated fast glycolytic fibers, Intermediate endplates supplied fast oxidative glycolytic fibers, and Simple endplates served slow oxidative fibers. In response to colchicine, the endplates of the slow muscles sprouted more than those of fast muscles while the innervation of slow fiber types sprouted less than that of fast fiber types. Furthermore, intranodal sprouts were more prevalent in slow muscles and ultraterminal sprouts more numerous in fast muscles whereas intranodal sprouts predominated on fast fiber types and ultraterminal sprouts were characteristic of slow fiber types. These apparently contradictory results were reconciled when it was noted that soleus endplates were mostly Complex and Intermediate, and the extensor digitorum longus contained more Simple endplates. Thus, consistency of sprouting patterns among endplate types of the 3 muscles was recognized when the pre-existing branching patterns were considered. This indicated that the patterns of sprouting were determined by the motor neurons rather than the muscle fibers. The observed sprouting responses supported the hypothesis that colchicine treatment of motor axons caused muscle fibers to elaborate a diffusible sprout-inducing factor.