HBO suppresses Nogo-A, Ng-R, or RhoA expression in the cerebral cortex after global ischemia

Selective temporal and regional alterations of Nogo-A and small proline-rich repeat protein 1A (SPRR1A) but not Nogo-66 receptor (NgR) occur following traumatic brain injury in the rat

Axons show a poor regenerative capacity following traumatic central nervous system (CNS) injury, partly due to the expression of inhibitors of axonal outgrowth, of which Nogo-A is considered the most important. We evaluated the acute expression of Nogo-A, the Nogo-66 receptor (NgR) and the novel small proline-rich repeat protein 1A (SPRR1A, previously undetected in brain), following experimental lateral fluid percussion (FP) brain injury in rats. Immunofluorescence with antibodies against Nogo-A, NgR and SPRR1A was combined with antibodies against the neuronal markers NeuN and microtubule-associated protein (MAP)-2 and the oligodendrocyte marker RIP, while Western blot analysis was performed for Nogo-A and NgR. Brain injury produced a significant increase in Nogo-A expression in injured cortex, ipsilateral external capsule and reticular thalamus from days 1-7 post-injury (P < 0.05) compared to controls. Increased expression of Nogo-A was observed in both RIP- and NeuN positive (+) cells in the ipsilateral cortex, in NeuN (+) cells in the CA3 region of the hippocampus and reticular thalamus and in RIP (+) cells in white matter tracts. Alterations in NgR expression were not observed following traumatic brain injury (TBI). Brain injury increased the extent of SPRR1A expression in the ipsilateral cortex and the CA3 at all post-injury time-points in NeuN (+) cells. The marked increases in Nogo-A and SPRR1A in several important brain regions suggest that although inhibitors of axonal growth may be upregulated, the injured brain is also capable of expressing proteins promoting axonal outgrowth following TBI.

Hyperbaric oxygen therapy of cerebral ischemia

BACKGROUND

Hyperbaric oxygen (HBO) therapy of cerebral ischemia has been evaluated in a number of human and animal studies; however, there is presently no consensus on its efficacy.

METHODS

We present a review of animal and human studies on HBO therapy of cerebral ischemia as well as present potential mechanisms of action of HBO.

RESULTS

Animal studies of HBO have shown promise by reducing infarct size and improving neurologic outcome. HBO has also been shown to inhibit inflammation and apoptosis after cerebral ischemia. Early reports in humans also suggested benefit in stroke patients treated with HBO. Recent randomized, controlled human studies, however, have not shown benefit, although all were limited by small sample size. Important differences between animal and human studies suggest HBO might be more effective in stroke within the first few hours and at a pressure of 2-3 ATA.

CONCLUSIONS

The clinical usefulness of HBO in the treatment of cerebral ischemia is not yet certain. Attention to emerging pathophysiologic data should be taken into consideration in design of any future clinical trials of HBO in acute ischemic stroke.

Injury reactive myelin/oligodendrocyte-derived axon growth inhibition in the adult mammalian central nervous system

Myelin inhibition is considered a constitutive, static, repulsive barrier not reactive to a central nervous system (CNS) lesion. However, recent evidence underlines the existence of considerable add-on axon growth inhibition upon CNS injury. This postlesional, reactive myelin/oligodendrocyte-derived inhibition will require the development of novel screening approaches and therapeutic reagents to promote axonal regeneration.

Mechanisms of hyperbaric oxygen and neuroprotection in stroke

Cerebral vascular diseases, such as neonatal encephalopathy and focal or global cerebral ischemia, all result in reduction of blood flow to the affected regions, and cause hypoxia-ischemia, disorder of energy metabolism, activation of pathogenic cascades, and eventual cell death. Due to a narrow therapeutic window for neuroprotection, few effective therapies are available, and prognosis for patients with these neurological injuries remains poor. Hyperbaric oxygen (HBO) has been used as a primary or adjunctive therapy over the last 50 years with controversial results, both in experimental and clinical studies. In addition, the mechanisms of HBO on neuroprotection remain elusive. Early applications of HBO within a therapeutic window of 3-6h or delayed but repeated administration of HBO can either salvage injured neuronal tissues or promote neurobehavioral functional recovery. This review explores the discrepancies between experimental and clinical observations of HBO, focusing on its therapeutic window in brain injuries, and discusses the potential mechanisms of HBO neuroprotection.

Injury-related dynamic myelin/oligodendrocyte axon-outgrowth inhibition in the central nervous system

CONTEXT

By contrast with the glial scar, myelin was considered a constitutive static inhibitory barrier unreactive to lesions in the central nervous system (CNS). However, recent results suggest considerable add-on inhibition of myelin as a result of CNS injury. Furthermore, catastrophic events cause morphological and biochemical changes in the axon itself. This results in the accumulation of cytoskeleton components and intraaxonal transported proteins paralleled by extensive membrane remodelling at the axonal tip (a process called axotomy) which might modify the axonal response to its inhibitory environment.

STARTING POINT

Ji-Eun Kim and colleagues recently reported an axonal subpopulation with a different capacity to respond to myelin inhibitors (Neuron 2004; 44: 439-51). Axonal specificity but also evidence for injury reactivity summarised here challenges our understanding of axon-growth inhibition in the injured CNS. This might be due to (i) qualitative and quantitative enrichment of the periaxonal environment by myelin/oligodendrocytes, (ii) increased axonal sensitivity to its inhibitory environment, and (iii) axons and lesion-induced, altered axonal signalling. WHERE NEXT? Postlesional reactive inhibition of myelin or the oligodendrocyte necessitates the development of novel screening approaches and therapeutic agents to promote axonal regeneration. Moreover, we need to improve our understanding of the pathophysiology of the lesion to find more efficient experimental strategies to restore neurological function.

The differential expression patterns of messenger RNAs encoding Nogo-A and Nogo-receptor in the rat central nervous system

Nogo-A and Nogo-receptor have been considered to play pivotal roles in controlling axonal regeneration and neuronal plasticity. We investigated the total distribution of Nogo-A and Nogo-receptor mRNAs in the adult rat central nervous system using in situ hybridization histochemistry. Nogo-A is abundantly expressed in both neurons and oligodendrocytes throughout the central nervous system. Interestingly, we could not find any neuron which lacks Nogo-A mRNA expression, indicating that Nogo-A mRNA is universally expressed in all neurons. In contrast, Nogo-R mRNA expression was very restricted. Nogo-R mRNA was expressed in the olfactory bulb, hippocampus, tentia tecta, some amygdala nuclei, cerebral cortex, some thalamic nuclei, medial habenular, whereas we could not detect it in the other regions. Interestingly, we did not detect Nogo-R mRNA in monoaminergic neurons, which are known to have high regenerative capacity, in the substantia nigra, ventral tegmental area, locus caeruleus, and raphe nuclei. In addition, although neurons in the reticular thalamus and cerebellar nuclei are also known to show high capacity for regeneration, Nogo-R mRNA was not detected there. These data indicate that Nogo-A and Nogo-R mRNAs were differentially expressed in the central nervous system, and suggest that the lack of Nogo-R expression in a given neuron might be necessary to keep its high regenerative capacity.

Nogo-A and nogo receptor expression in demyelinating lesions of multiple sclerosis

A myelin-associated neurite outgrowth inhibitor, Nogo-A, plays a key role in inhibition of axonal regeneration following injury and ischemia in the central nervous system (CNS). Because axonal injury is a pathologic hallmark of multiple sclerosis (MS), we have investigated the expression of Nogo-A and its receptor NgR in four MS and 12 non-MS control brains by immunohistochemistry. Nogo-A expression was markedly upregulated in surviving oligodendrocytes at the edge of chronic active demyelinating lesions of MS and ischemic lesions of acute and old cerebral infarction, whereas NgR expression was greatly enhanced in reactive astrocytes and microglia/macrophages in these lesions when compared with their expression in the brains of neurologically normal controls. Nogo-A and NgR were also identified in a subpopulation of neurons. In contrast, Nogo-A was undetectable in reactive astrocytes and microglia/macrophages and NgR was not expressed on oligodendrocytes in any cases examined. Western blot analysis and double labeling immunocytochemistry identified the constitutive expression of NgR in cultured human astrocytes. These results suggest that Nogo-A expressed on oligodendrocytes might interact with NgR presented by reactive astrocytes and microglia/macrophages in active demyelinating lesions of MS, although biologic effects caused by Nogo-A/NgR interaction among glial cells remain unknown.

Multiple effects of hyperbaric oxygen on the expression of HIF-1 alpha and apoptotic genes in a global ischemia-hypotension rat model

Hypoxia-inducible factor-1alpha (HIF-1alpha) is a transcription factor specifically activated by hypoxia. Activation of proapoptotic caspase-9 and caspase-3 pathways, by binding with tumor suppressor p53, HIF-1alpha could lead to harmful actions such as apoptosis. We examined whether increasing oxygen levels by hyperbaric oxygen (HBO) offers neuroprotection, at least partially by suppression of HIF-1alpha and apoptotic genes. Male SD rats (n = 78) were randomly divided into 13 groups: 1 sham group, 6 groups of global ischemia-hypotension (GI), and 6 groups of HBO treatment after global ischemia-hypotension (GI + HBO). HBO (3 ATA for 2 h) was applied at 1 h after global ischemia-hypotension. Rats were sacrificed at 6, 12, 24, 48, and 96 h and 7 days. Global ischemia-hypotension (10 min ischemia, 30-35 mm Hg) produced a marked increase of HIF-1alpha expressions in the hippocampus and cortex at 6 h and peaked at 48-96 h. The expressions of p53, caspase-9, and caspase-3 were all increased in a similar time course. These molecular changes were accompanied by massive cell loss in the hippocampal regions and to a lesser degree in the cortex, with features of apoptosis. HBO treatment reduced expressions of HIF-1alpha, p53, caspase-9, and caspase-3 and decreased cell death. The protein levels of proapoptotic caspase-8 and antiapoptotic bcl-2 were increased after global ischemia-hypotension and HBO potentiated the expression of caspase-8 and decreased expression of bcl-2. These results indicate that HBO has multiple actions on apoptotic genes even though the overall effect of HBO was decreased HIF-1alpha expression and reduced apoptosis after global ischemia-hypotension.

Transient exposure of rat pups to hyperoxia at normobaric and hyperbaric pressures does not cause retinopathy of prematurity

We have shown that hyperoxia reduces brain damage in a rat model of hypoxia-ischemia. The purpose of this study was to examine the possibility of hyperoxia in inducing vision-threatening retinopathy. Two different experiments were conducted in this study. PART 1: seven-day-old rat pups were subjected to unilateral carotid artery ligation followed by 2 h of hypoxia (8% O2 at 37 degrees C). Pups were treated with 100% oxygen at 1 ATA, 1.5 ATA, and 3.0 ATA for a duration of 1 h. PART 2: Newborn rat pups were exposed to 100% oxygen at 1, 1.5, or 3.0 ATA for 1 h, the same treatment protocol used for brain protection after hypoxia-ischemia. Retinopathy was evaluated by the degree of neovascularization (measuring retinal vascular density), by the structural abnormalities (histology) in the retina, and by the expression of hypoxia-hyperoxia sensitive proteins including hypoxia-inducible factor-1alpha (HIF-1alpha) and vascular endothelial growth factor (VEGF) at 24 h, 1, 2, and 10 weeks after hyperoxia exposure. Hyperoxic treatment at all pressures administered significantly reduced the hypoxia-ischemic-induced reduction in brain weight. Retinal vascular density measurements revealed no signs of neovascularization after hyperoxia exposure. There were also no abnormalities in the structure of the retina and no changes in the protein expression of HIF-1alpha and VEGF following hyperoxia exposure. Exposure to hyperoxia for 1 h at normobaric or hyperbaric pressures did not result in the structural changes or abnormal vascularization that is associated with retinopathy of prematurity, suggesting that hyperoxia is a safe treatment for hypoxic newborn infants.

Inter- and intracellular interactions of Nogo: new findings and hypothesis

The molecule Nogo has captured the imagination of many as a possible key player, and therefore therapeutic target, in the pathological settings of central nervous system (CNS) injury and degenerative pathology. Found in both glial cells and neurons, the endogenous, physiological role of Nogo is as yet unknown. Recently reported targeted disruption of the Nogo gene did not result in any obvious neuro-anatomical or neurological phenotype. Compared with wild-type mice, Nogo-deficient mice also did not exhibit a truly convincing enhancement in their ability to regenerate CNS neurons upon injury. Does the molecule have any important physiological function at all? Other recent discoveries of new interacting partners of Nogo at the mitochondria and the CNS paranode suggest intriguing links to the modulation of apoptosis and developmental organization or signalling at the axoglial junction.