Xiangru Xu

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.

AGEMAP: A Gene Expression Database for Aging in Mice

We present the AGEMAP (Atlas of Gene Expression in Mouse Aging Project) gene expression database, which is a resource that catalogs changes in gene expression as a function of age in mice. The AGEMAP database includes expression changes for 8,932 genes in 16 tissues as a function of age. We found great heterogeneity in the amount of transcriptional changes with age in different tissues. Some tissues displayed large transcriptional differences in old mice, suggesting that these tissues may contribute strongly to organismal decline. Other tissues showed few or no changes in expression with age, indicating strong levels of homeostasis throughout life. Based on the pattern of age-related transcriptional changes, we found that tissues could be classified into one of three aging processes: (1) a pattern common to neural tissues, (2) a pattern for vascular tissues, and (3) a pattern for steroid-responsive tissues. We observed that different tissues age in a coordinated fashion in individual mice, such that certain mice exhibit rapid aging, whereas others exhibit slow aging for multiple tissues. Finally, we compared the transcriptional profiles for aging in mice to those from humans, flies, and worms. We found that genes involved in the electron transport chain show common age regulation in all four species, indicating that these genes may be exceptionally good markers of aging. However, we saw no overall correlation of age regulation between mice and humans, suggesting that aging processes in mice and humans may be fundamentally different.

Leptin-mediated cell survival signaling in hippocampal neurons mediated by JAK STAT3 and mitochondrial stabilization.

Leptin plays a pivotal role in the regulation of energy homeostasis and metabolism, primarily by acting on neurons in the hypothalamus that control food intake. However, leptin receptors are more widely expressed in the brain suggesting additional, as yet unknown, functions of leptin. Here we show that both embryonic and adult hippocampal neurons express leptin receptors coupled to activation of STAT3 and phosphatidylinositol 3-kinase-Akt signaling pathways. Leptin protects hippocampal neurons against cell death induced by neurotrophic factor withdrawal and excitotoxic and oxidative insults. The neuroprotective effect of leptin is antagonized by the JAK2-STAT3 inhibitor AG-490, STAT3 decoy DNA, and phosphatidylinositol 3-kinase/Akt inhibitors but not by an inhibitor of MAPK. Leptin induces the production of manganese superoxide dismutase and the anti-apoptotic protein Bcl-xL, and stabilizes mitochondrial membrane potential and lessens mitochondrial oxidative stress. Leptin receptor-deficient mice (db/db mice) are more vulnerable to seizure-induced hippocampal damage, and intraventricular administration of leptin protects neurons against seizures. By enhancing mitochondrial resistance to apoptosis and excitotoxicity, our findings suggest that leptin signaling serves a neurotrophic function in the developing and adult hippocampus.

Gene expression atlas of the mouse central nervous system: impact and interactions of age, energy intake and gender.

BackgroundThe structural and functional complexity of the mammalian central nervous system (CNS) is organized and modified by complicated molecular signaling processes that are poorly understood.ResultsWe measured transcripts of 16,896 genes in 5 CNS regions from cohorts of young, middle-aged and old male and female mice that had been maintained on either a control diet or a low energy diet known to retard aging. Each CNS region (cerebral cortex, hippocampus, striatum, cerebellum and spinal cord) possessed its own unique transcriptome fingerprint that was independent of age, gender and energy intake. Less than 10% of genes were significantly affected by age, diet or gender, with most of these changes occurring between middle and old age. The transcriptome of the spinal cord was the most responsive to age, diet and gender, while the striatal transcriptome was the least responsive. Gender and energy restriction had particularly robust influences on the hippocampal transcriptome of middle-aged mice. Prominent functional groups of age- and energy-sensitive genes were those encoding proteins involved in DNA damage responses (Werner and telomere-associated proteins), mitochondrial and proteasome functions, cell fate determination (Wnt and Notch signaling) and synaptic vesicle trafficking.ConclusionMouse CNS transcriptomes responded to age, energy intake and gender in a regionally distinctive manner. The systematic transcriptome dataset also provides a window into mechanisms of age-, diet- and sex-related CNS plasticity and vulnerability.

Paroxetine Retards Disease Onset and Progression in Huntingtin Mutant Mice

We report that administration of paroxetine, a widely prescribed antidepressant drug that acts by inhibiting reuptake of the neurotransmitter serotonin, suppresses the neurodegenerative process and increases the survival of huntingtin mutant mice, an animal model of Huntingtons disease (HD). Paroxetine attenuated motor dysfunction and body weight loss and improved glucose metabolism in the HD mice. Paroxetine was beneficial when treatment was initiated before or after the onset of motor dysfunction, suggesting a potential for such antidepressant drugs in the treatment of presymptomatic and symptomatic HD patients.

Homocysteic acid induces intraneuronal accumulation of neurotoxic Aβ42: Implications for the pathogenesis of Alzheimer's disease

The causes of neuronal dysfunction and degeneration in Alzheimers disease (AD) are not fully understood, but increased production of neurotoxic forms of amyloid β‐peptide‐42 (Aβ42) seems of major importance. Large extracellular deposits of aggregated Aβ42 (plaques) is a diagnostic feature of AD, but Aβ42 may be particularly cytotoxic when it accumulates inside neurons. The factors that may promote the intracellular accumulation of Aβ42 in AD are unknown, but recent findings suggest that individuals with elevated homocysteine levels are at increased risk for AD. We show that homocysteic acid (HA), an oxidized metabolite of homocysteine, induces intraneuronal accumulation of a Aβ42 that is associated with cytotoxicity. The neurotoxicity of HA can be attenuated by an inhibitor of γ‐secretase, the enzyme activity that generates Aβ42, suggesting a key role for intracellular Aβ42 accumulation in the neurotoxic action of HA. Concentrations of HA in cerebrospinal fluid (CSF) were similar in AD and control subjects. CSF homocysteine levels were elevated significantly in AD patients, however, and homocysteine exacerbated HA‐induced neurotoxicity, suggesting a role for HA in the pathogenic action of elevated homocysteine levels in AD. These findings suggest that the intracellular accumulation of Aβ42 plays a role in the neurotoxic action of HA, and suggest a potential therapeutic benefit of agents that modify the production and neurotoxic actions of HA and homocysteine.

Conservation and variation of gene regulation in embryonic stem cells assessed by comparative genomics

We have examined the gene structure and regulatory regions of octamer-binding transcription factor 3/4 (Oct 3/4), sex determining region Y box 2 (Sox2), signal transducer and activator of transcription 3 (Stat3), embryonal stem cell-specific gene 1 (ESG), nanog homeobox (Nanog), and several other genes highly expressed in embryonic stem (ES) cells across different species. Our analysis showed that ES cell-expressed Ras (ERAS) was orthologous to a human pseudogene Harvey Ras (HRASP) and that the promoter and other regulatory sequences were highly divergent. No ortholog of (ES) cell-derived homeobox containing gene (Ehox) could be identified in human, and the closest paralogs PEPP gene subfamily 1 (PEPP1), PEPP2, and extraembryonic, spermatogenesis, homeobox 1 (Esx1) were not expressed by ES cells and shared little homology. The Sox2 promoter was the most conserved across species and the Oct3/4 promoter region showed significant homology particularly in the distal enhancer active in ES cells. Analysis suggested common and divergent pathways of regulation. Conserved Oct3/4 and Sox2 co-binding domains were identified in most ES expressed genes, highlighting the importance of this transcriptional pathway. Conserved fibroblast growth factor response element sites were identified in regulatory regions, suggesting a potential parallel pathway for regulation by FGFs. A central role of Stat3 activation in self-renewal and in a regulatory feedback loop was suggested by the identification of the conserved binding sites in most pathways. Although most pathways were evolutionarily conserved, promoters and genomic structure of the leukemia inhibitory factor (LIF) pathway components were divergent, likely explaining the differential requirement of LIF for human and rodent cells. Our analysis further suggested that the Nanog regulatory pathway was relatively independent of the LIF/Oct pathway and may interact with the Nodal/transforming growth factor-β pathway. These results provide a framework for examining the current reported differences between rodent and human ES cells and define targets for future perturbation studies.

TRF2 dysfunction elicits DNA damage responses associated with senescence in proliferating neural cells and differentiation of neurons

Telomeres are specialized structures at the ends of chromosomes that consist of tandem repeats of the DNA sequence TTAGGG and several proteins that protect the DNA and regulate the plasticity of the telomeres. The telomere‐associated protein TRF2 (telomeric repeat binding factor 2) is critical for the control of telomere structure and function; TRF2 dysfunction results in the exposure of the telomere ends and activation of ATM (ataxia telangiectasin mutated)‐mediated DNA damage response. Recent findings suggest that telomere attrition can cause senescence or apoptosis of mitotic cells, but the function of telomeres in differentiated neurons is unknown. Here, we examined the impact of telomere dysfunction via TRF2 inhibition in neurons (primary embryonic hippocampal neurons) and mitotic neural cells (astrocytes and neuroblastoma cells). We demonstrate that telomere dysfunction induced by adenovirus‐mediated expression of dominant‐negative TRF2 (DN‐TRF2) triggers a DNA damage response involving the formation of nuclear foci containing phosphorylated histone H2AX and activated ATM in each cell type. In mitotic neural cells DN‐TRF2 induced activation of both p53 and p21 and senescence (as indicated by an up‐regulation of β‐galactosidase). In contrast, in neurons DN‐TRF2 increased p21, but neither p53 nor β‐galactosidase was induced. In addition, TRF2 inhibition enhanced the morphological, molecular and biophysical differentiation of hippocampal neurons. These findings demonstrate divergent molecular and physiological responses to telomere dysfunction in mitotic neural cells and neurons, indicate a role for TRF2 in regulating neuronal differentiation, and suggest a potential therapeutic application of inhibition of TRF2 function in the treatment of neural tumors.

Numb-mediated neurite outgrowth is isoform-dependent, and requires activation of voltage-dependent calcium channels

Numb is an evolutionarily conserved protein that controls the differentiation of neuronal progenitor cells by unknown mechanisms. Here we report that the neural cells expressing Numb isoforms with short phosphotyrosine-binding (SPTB) domain undergo extensive neurite outgrowth, an effect that can be blocked by voltage-gated Ca2+ channel (VGCC) inhibitor or by Ca2+ chelator. In contrast, tyrosine kinase inhibitor, genistein, and selective receptor tyrosine kinase (TrkA) inhibitor, K252alpha did not affect SPTB Numb-mediated neurite outgrowth. MAP kinase inhibitor, PD98059 partially reduced SPTB Numb-mediated neurite outgrowth. Cells expressing SPTB Numbs exhibit increased whole-cell Ca2+ current densities (ICa) which can be prevented by preincubation of either nifedipine or PD98095. Cells expressing LPTB Numbs expressed little ICa (density) and were not able to grow neurites. Our results indicate that Ca2+ influx through VGCC may be required for SPTB Numb-mediated neurite outgrowth, suggesting that Numb promotes neuronal differentiation by a mechanism involving PTB domain-specific regulation of Ca2+ influx and MAP kinase activation.

Dietary restriction mitigates cocaine-induced alterations of olfactory bulb cellular plasticity and gene expression, and behavior.

J. Neurochem. (2010) 114, 323–334.