Aiwu Cheng

Pivotal role for neuronal Toll-like receptors in ischemic brain injury and functional deficits.

The innate immune system senses the invasion of pathogenic microorganisms and tissue injury through Toll-like receptors (TLR), a mechanism thought to be limited to immune cells. We now report that neurons express several TLRs, and that the levels of TLR2 and -4 are increased in neurons in response to IFN-γ stimulation and energy deprivation. Neurons from both TLR2 knockout and -4 mutant mice were protected against energy deprivation-induced cell death, which was associated with decreased activation of a proapoptotic signaling cascade involving jun N-terminal kinase and the transcription factor AP-1. TLR2 and -4 expression was increased in cerebral cortical neurons in response to ischemia/reperfusion injury, and the amount of brain damage and neurological deficits caused by a stroke were significantly less in mice deficient in TLR2 or -4 compared with WT control mice. Our findings establish a proapoptotic signaling pathway for TLR2 and -4 in neurons that may render them vulnerable to ischemic death.

Mitochondria in Neuroplasticity and Neurological Disorders

Mitochondrial electron transport generates the ATP that is essential for the excitability and survival of neurons, and the protein phosphorylation reactions that mediate synaptic signaling and related long-term changes in neuronal structure and function. Mitochondria are highly dynamic organelles that divide, fuse, and move purposefully within axons and dendrites. Major functions of mitochondria in neurons include the regulation of Ca(2+) and redox signaling, developmental and synaptic plasticity, and the arbitration of cell survival and death. The importance of mitochondria in neurons is evident in the neurological phenotypes in rare diseases caused by mutations in mitochondrial genes. Mitochondria-mediated oxidative stress, perturbed Ca(2+) homeostasis, and apoptosis may also contribute to the pathogenesis of prominent neurological diseases including Alzheimers, Parkinsons, and Huntingtons diseases; stroke; amyotrophic lateral sclerosis; and psychiatric disorders. Advances in understanding the molecular and cell biology of mitochondria are leading to novel approaches for the prevention and treatment of neurological disorders.

Nitric oxide acts in a positive feedback loop with BDNF to regulate neural progenitor cell proliferation and differentiation in the mammalian brain.

Nitric oxide (NO) is believed to act as an intercellular signal that regulates synaptic plasticity in mature neurons. We now report that NO also regulates the proliferation and differentiation of mouse brain neural progenitor cells (NPCs). Treatment of dissociated mouse cortical neuroepithelial cluster cell cultures with the NO synthase inhibitor L-NAME or the NO scavenger hemoglobin increased cell proliferation and decreased differentiation of the NPCs into neurons, whereas the NO donor sodium nitroprusside inhibited NPC proliferation and increased neuronal differentiation. Brain-derived neurotrophic factor (BDNF) reduced NPC proliferation and increased the expression of neuronal NO synthase (nNOS) in differentiating neurons. The stimulatory effect of BDNF on neuronal differentation of NPC was blocked by L-NAME and hemoglobin, suggesting that NO produced by the latter cells inhibited proliferation and induced neuronal differentiation of neighboring NPCs. A similar role for NO in regulating the switch of neural stem cells from proliferation to differentiation in the adult brain is suggested by data showing that NO synthase inhibition enhances NPC proliferation and inhibits neuronal differentiation in the subventricular zone of adult mice. These findings identify NO as a paracrine messenger stimulated by neurotrophin signaling in newly generated neurons to control the proliferation and differentiation of NPC, a novel mechanism for the regulation of developmental and adult neurogenesis.

Membrane properties of rat embryonic multipotent neural stem cells

We have characterized several potential stem cell markers and defined the membrane properties of rat fetal (E10.5) neural stem cells (NSC) by immunocytochemistry, electrophysiology and microarray analysis. Immunocytochemical analysis demonstrates specificity of expression of Sox1, ABCG2/Bcrp1, and shows that nucleostemin labels both progenitor and stem cell populations. NSCs, like hematopoietic stem cells, express high levels of aldehyde dehydrogenase (ALDH) as assessed by Aldefluor labeling. Microarray analysis of 96 transporters and channels showed that Glucose transporter 1 (Glut1/Slc2a1) expression is unique to fetal NSCs or other differentiated cells. Electrophysiological examination showed that fetal NSCs respond to acetylcholine and its agonists, such as nicotine and muscarine. NSCs express low levels of tetrodotoxin (TTX) sensitive and insensitive sodium channels and calcium channels while expressing at least three kinds of potassium channels. We find that gap junction communication is mediated by connexin (Cx)43 and Cx45, and is essential for NSC survival and proliferation. Overall, our results show that fetal NSCs exhibit a unique signature that can be used to determine their location and assess their ability to respond to their environment.

Mitochondria and neuroplasticity

The production of neurons from neural progenitor cells, the growth of axons and dendrites and the formation and reorganization of synapses are examples of neuroplasticity. These processes are regulated by cell-autonomous and intercellular (paracrine and endocrine) programs that mediate responses of neural cells to environmental input. Mitochondria are highly mobile and move within and between subcellular compartments involved in neuroplasticity (synaptic terminals, dendrites, cell body and the axon). By generating energy (ATP and NAD+), and regulating subcellular Ca2+ and redox homoeostasis, mitochondria may play important roles in controlling fundamental processes in neuroplasticity, including neural differentiation, neurite outgrowth, neurotransmitter release and dendritic remodelling. Particularly intriguing is emerging data suggesting that mitochondria emit molecular signals (e.g. reactive oxygen species, proteins and lipid mediators) that can act locally or travel to distant targets including the nucleus. Disturbances in mitochondrial functions and signalling may play roles in impaired neuroplasticity and neuronal degeneration in Alzheimers disease, Parkinsons disease, psychiatric disorders and stroke.

Gamma secretase–mediated Notch signaling worsens brain damage and functional outcome in ischemic stroke

Mice transgenic for antisense Notch and normal mice treated with inhibitors of the Notch-activating enzyme γ-secretase showed reduced damage to brain cells and improved functional outcome in a model of focal ischemic stroke. Notch endangers neurons by modulating pathways that increase their vulnerability to apoptosis, and by activating microglial cells and stimulating the infiltration of proinflammatory leukocytes. These findings suggest that Notch signaling may be a therapeutic target for treatment of stroke and related neurodegenerative conditions.

Toll-like receptor-4 mediates neuronal apoptosis induced by amyloid β-peptide and the membrane lipid peroxidation product 4-hydroxynonenal

The innate immune system senses the invasion of pathogenic microorganisms and tissue injury through Toll-like receptors (TLR), a mechanism thought to be limited to immune cells. We recently found that neurons express several TLRs, and that the levels of TLR2 and TLR4 are increased in neurons in response to energy deprivation. Here we report that TLR4 expression increases in neurons when exposed to amyloid beta-peptide (Abeta1-42) or the lipid peroxidation product 4-hydroxynonenal (HNE). Neuronal apoptosis triggered by Abeta and HNE was mediated by jun N-terminal kinase (JNK); neurons from TLR4 mutant mice exhibited reduced JNK and caspase-3 activation and were protected against apoptosis induced by Abeta and HNE. Levels of TLR4 were decreased in inferior parietal cortex tissue specimens from end-stage AD patients compared to aged-matched control subjects, possibly as the result of loss of neurons expressing TLR4. Our findings suggest that TLR4 signaling increases the vulnerability of neurons to Abeta and oxidative stress in AD, and identify TLR4 as a potential therapeutic target for AD.

Intravenous immunoglobulin (IVIG) protects the brain against experimental stroke by preventing complement-mediated neuronal cell death

Stroke is among the three leading causes of death worldwide and the most frequent cause of permanent disability. Brain ischemia induces an inflammatory response involving activated complement fragments. Here we show that i.v. Ig (IVIG) treatment, which scavenges complement fragments, protects brain cells against the deleterious effects of experimental ischemia and reperfusion (I/R) and prevents I/R-induced mortality in mice. Animals administered IVIG either 30 min before ischemia or after 3 h of reperfusion exhibited a 50–60% reduction of brain infarct size and a 2- to 3-fold improvement of the functional outcome. Even a single low dose of IVIG given after stroke was effective. IVIG was protective in the nonreperfusion model of murine stroke as well and did not exert any peripheral effects. Human IgG as well as intrinsic murine C3 levels were significantly higher in the infarcted brain region compared with the noninjured side, and their physical association was demonstrated by immuno-coprecipitation. C5-deficient mice were significantly protected from I/R injury compared with their wild-type littermates. Exposure of cultured neurons to oxygen/glucose deprivation resulted in increased levels of C3 associated with activation of caspase 3, a marker of apoptosis; both signals were attenuated with IVIG treatment. Our data suggest a major role for complement-mediated cell death in ischemic brain injury and the prospect of using IVIG in relatively low doses as an interventional therapy for stroke.

Caloric restriction and intermittent fasting alter spectral measures of heart rate and blood pressure variability in rats

Dietary restriction (DR) has been shown to increase life span, delay or prevent age‐associated diseases, and improve functional and metabolic cardiovascular risk factors in rodents and other species. To investigate the effects of DR on beat‐to‐beat heart rate and diastolic blood pressure variability (HRV and DPV) in male Sprague‐Dawley rats, we implanted telemetric transmitters and animals were maintained on either intermittent fasting (every other day feeding) or calorie‐restricted (40% caloric reduction) diets. Using power spectral analysis, we evaluated the temporal profiles of the low‐and high‐frequency oscillatory components in heart rate and diastolic blood pressure signals to assess cardiac autonomic activity. Body weight, heart rate, and systolic and diastolic blood pressure were all found to decrease in response to DR. Both methods of DR produced decreases in the low‐frequency component of DPV spectra, a marker for sympathetic tone, and the high‐frequency component of HRV spectra, a marker for parasympathetic activity, was increased. These parameters required at least 1 month to become maximal, but returned toward base‐line values rapidly once rats resumed ad libitum diets. These results suggest an additional cardiovascular benefit of DR that merits further studies of this potential effect in humans.‐Mager, D. E., Wan, R., Brown, M., Cheng, A., Wareski, P., Abernethy, D. R., Mattson, M. P. Caloric restriction and intermittent fasting alter spectral measures of heart rate and blood pressure variability in rats. FASEB J. 20, 631–637 (2006)

Age and Energy Intake Interact to Modify Cell Stress Pathways and Stroke Outcome

Age and excessive energy intake/obesity are risk factors for cerebrovascular disease, but it is not known if and how these factors affect the extent of brain damage and outcome in ischemic stroke. We therefore determined the interactions of age and energy intake on the outcome of ischemic brain injury, and elucidated the underlying mechanisms.