Molecular and Cellular Mechanisms of Ischemia-Induced Neuronal Death

Identification of the common neurobiological process disturbed in genetic and non-genetic models for autism spectrum disorders

Autism spectrum disorders (ASD) are neurodevelopmental disorders. Genetic factors, along with non-genetic triggers, have been shown to play a causative role. Despite the various causes, a triad of common symptoms defines individuals with ASD; pervasive social impairments, impaired social communication, and repeated sensory-motor behaviors. Therefore, it can be hypothesized that different genetic and environmental factors converge on a single hypothetical neurobiological process that determines these behaviors. However, the cellular and subcellular signature of this process is, so far, not well understood. Here, we performed a comparative study using "omics" approaches to identify altered proteins and, thereby, biological processes affected in ASD. In this study, we mined publicly available repositories for genetic mouse model data sets, identifying six that were suitable, and compared them with in-house derived proteomics data from prenatal zinc (Zn)-deficient mice, a non-genetic mouse model with ASD-like behavior. Findings derived from these comparisons were further validated using in vitro neuronal cell culture models for ASD. We could show that a protein network, centered on VAMP2, STX1A, RAB3A, CPLX2, and AKAP5, is a key convergence point mediating synaptic vesicle release and recycling, a process affected across all analyzed models. Moreover, we demonstrated that Zn availability has predictable functional effects on synaptic vesicle release in line with the alteration of proteins in this network. In addition, drugs that target kinases, reported to regulate key proteins in this network, similarly impacted the proteins' levels and distribution. We conclude that altered synaptic stability and plasticity through abnormal synaptic vesicle dynamics and function may be the common neurobiological denominator of the shared behavioral abnormalities in ASD and, therefore, a prime drug target for developing therapeutic strategies.

Comparative Transcriptome Analysis in Monocyte-Derived Macrophages of Asymptomatic GBA Mutation Carriers and Patients with GBA-Associated Parkinson's Disease

Mutations of the GBA gene, encoding for lysosomal enzyme glucocerebrosidase (GCase), are the greatest genetic risk factor for Parkinson’s disease (PD) with frequency between 5% and 20% across the world. N370S and L444P are the two most common mutations in the GBA gene. PD carriers of severe mutation L444P in the GBA gene is characterized by the earlier age at onset compared to N370S. Not every carrier of GBA mutations develop PD during one’s lifetime. In the current study we aimed to find common gene expression signatures in PD associated with mutation in the GBA gene (GBA-PD) using RNA-seq. We compared transcriptome of monocyte-derived macrophages of 5 patients with GBA-PD (4 L444P/N, 1 N370S/N) and 4 asymptomatic GBA mutation carriers (GBA-carriers) (3 L444P/N, 1 N370S/N) and 4 controls. We also conducted comparative transcriptome analysis for L444P/N only GBA-PD patients and GBA-carriers. Revealed deregulated genes in GBA-PD independently of GBA mutations (L444P or N370S) were involved in immune response, neuronal function. We found upregulated pathway associated with zinc metabolism in L444P/N GBA-PD patients. The potential important role of DUSP1 in the pathogenesis of GBA-PD was suggested.

In vitro ischemia triggers a transcriptional response to down-regulate synaptic proteins in hippocampal neurons

Transient global cerebral ischemia induces profound changes in the transcriptome of brain cells, which is partially associated with the induction or repression of genes that influence the ischemic response. However, the mechanisms responsible for the selective vulnerability of hippocampal neurons to global ischemia remain to be clarified. To identify molecular changes elicited by ischemic insults, we subjected hippocampal primary cultures to oxygen-glucose deprivation (OGD), an in vitro model for global ischemia that resulted in delayed neuronal death with an excitotoxic component. To investigate changes in the transcriptome of hippocampal neurons submitted to OGD, total RNA was extracted at early (7 h) and delayed (24 h) time points after OGD and used in a whole-genome RNA microarray. We observed that at 7 h after OGD there was a general repression of genes, whereas at 24 h there was a general induction of gene expression. Genes related with functions such as transcription and RNA biosynthesis were highly regulated at both periods of incubation after OGD, confirming that the response to ischemia is a dynamic and coordinated process. Our analysis showed that genes for synaptic proteins, such as those encoding for PICK1, GRIP1, TARPγ3, calsyntenin-2/3, SAPAP2 and SNAP-25, were down-regulated after OGD. Additionally, OGD decreased the mRNA and protein expression levels of the GluA1 AMPA receptor subunit as well as the GluN2A and GluN2B subunits of NMDA receptors, but increased the mRNA expression of the GluN3A subunit, thus altering the composition of ionotropic glutamate receptors in hippocampal neurons. Together, our results present the expression profile elicited by in vitro ischemia in hippocampal neurons, and indicate that OGD activates a transcriptional program leading to down-regulation in the expression of genes coding for synaptic proteins, suggesting that the synaptic proteome may change after ischemia.

Epigenetic mechanisms in stroke and epilepsy

Epigenetic remodeling and modifications of chromatin structure by DNA methylation and histone modifications represent central mechanisms for the regulation of neuronal gene expression during brain development, higher-order processing, and memory formation. Emerging evidence implicates epigenetic modifications not only in normal brain function, but also in neuropsychiatric disorders. This review focuses on recent findings that disruption of chromatin modifications have a major role in the neurodegeneration associated with ischemic stroke and epilepsy. Although these disorders differ in their underlying causes and pathophysiology, they share a common feature, in that each disorder activates the gene silencing transcription factor REST (repressor element 1 silencing transcription factor), which orchestrates epigenetic remodeling of a subset of 'transcriptionally responsive targets' implicated in neuronal death. Although ischemic insults activate REST in selectively vulnerable neurons in the hippocampal CA1, seizures activate REST in CA3 neurons destined to die. Profiling the array of genes that are epigenetically dysregulated in response to neuronal insults is likely to advance our understanding of the mechanisms underlying the pathophysiology of these disorders and may lead to the identification of novel therapeutic strategies for the amelioration of these serious human conditions.

Eradicating the mediators of neuronal death with a fine-tooth comb

Ischemic tolerance is an evolutionarily conserved form of cerebral plasticity in which a brief period of cerebral ischemia (called ischemic preconditioning) confers transient tolerance to a subsequent ischemic challenge in the brain. Polycomb group proteins are gene-silencing factors that are abundant and widely distributed during embryogenesis and are essential to epigenetic cellular memory, pluripotency, and stem cell self-renewal. New insight into the molecular mechanisms underlying ischemic tolerance is highlighted by the finding that ischemic preconditioning activates polycomb proteins in mature neurons. Polycomb proteins act through epigenetic gene silencing to eradicate potential mediators of neuronal death and promote cellular arrest, enabling mature neurons to survive ischemic stroke.

Oestradiol and insulin-like growth factor-1 reduce cell loss after global ischaemia in middle-aged female rats

Whereas the ability of oestradiol and insulin-like growth factor (IGF)-1 to afford neuroprotection against ischaemia-induced neuronal death in young female and male rodents is well established, the impact of IGF-1 in middle-aged animals is largely unknown. The present study assessed the efficacy of oestradiol and IGF-1 with respect to reducing neuronal death after transient global ischaemia in middle-aged female rats after 8 weeks of hormone withdrawal. Rats were ovariohysterectomised and implanted 8 weeks later with an osmotic mini-pump delivering IGF-1 or saline into the lateral ventricle. Some rats also received physiological levels of oestradiol by subcutaneous pellet. Two weeks later, rats were subjected to global ischaemia or sham operation. Surviving hippocampal CA1 neurones were quantified. Ischaemia produced massive CA1 cell death compared to sham-operated animals, which was evident at 14 days. Significantly more neurones survived in animals treated with either oestradiol or IGF-1, but simultaneous treatment produced no additive effect. IGF-1, an endogenous growth factor, may be a clinically useful therapy in preventing human brain injury, with neuroprotective equivalence to oestradiol but without the harmful side-effects.

Molecular Mechanisms in the Pathogenesis and Treatment of Acute Ischemic Stroke

The management of acute ischemic stroke has not made significant strides since the introduction of recombinant tissue plasminogen activator (r-TPA) two decades ago. The use of other therapies, such as heparin, aspirin, dipyridamole, and/or clopidogrel, have only moderately aided in the treatment of this ischemic disease. Therefore, major medical innovative approaches are critically needed. Because of the side effects associated with r-TPA (specifically bleeding) and its limited 3-h therapeutic window, new studies using current developments encountered in the molecular biology of ischemia are being incorporated into the potential therapy of ischemic stroke. A review of the major advances in the field, including glutamate receptor blockade, magnesium infusion, inflammation blockade, apoptosis inhibition, and other therapies, is introduced with special emphasis on the molecular findings recognized as targets for a better and more effective treatment. As new therapies are being considered, the time of administration is becoming a central point of study for the application of novel therapeutic initiatives.

The endogenous inhibitor of Akt, CTMP, is critical to ischemia-induced neuronal death

Dysregulation of Akt signaling is important in a broad range of diseases that includes cancer, diabetes and heart disease. The role of Akt signaling in brain disorders is less clear. We found that global ischemia in intact rats triggered expression and activation of the Akt inhibitor CTMP (carboxyl-terminal modulator protein) in vulnerable hippocampal neurons and that CTMP bound and extinguished Akt activity and was essential to ischemia-induced neuronal death. Although ischemia induced a marked phosphorylation and nuclear translocation of Akt, phosphorylated Akt was not active in post-ischemic neurons, as assessed by kinase assays and phosphorylation of the downstream targets GSK-3beta and FOXO3A. RNA interference-mediated depletion of CTMP in a clinically relevant model of stroke restored Akt activity and rescued hippocampal neurons. Our results indicate that CTMP is important in the neurodegeneration that is associated with stroke and identify CTMP as a therapeutic target for the amelioration of hippocampal injury and cognitive deficits.

Ischemic preconditioning blocks BAD translocation, Bcl-xL cleavage, and large channel activity in mitochondria of postischemic hippocampal neurons

Transient forebrain or global ischemia induces delayed neuronal death in vulnerable CA1 pyramidal cells with many features of apoptosis. A brief period of ischemia, i.e., ischemic preconditioning, affords robust protection of CA1 neurons against a subsequent more prolonged ischemic challenge. Here we show that preconditioning acts via PI3K/Akt signaling to block the ischemia-induced cascade involving mitochondrial translocation of Bad, assembly of Bad with Bcl-x(L), cleavage of Bcl-x(L) to form its prodeath fragment, DeltaN-Bcl-x(L), activation of large-conductance channels in the mitochondrial outer membrane, mitochondrial release of cytochrome c and Smac/DIABLO (second mitochondria-derived activator of caspases/direct IAP-binding protein with low pI), caspase activation, and neuronal death. These findings show how preconditioning acts to prevent the release of cytochrome c and Smac/DIABLO from mitochondria and to preserve the integrity of the mitochondrial membrane. The specific PI3K inhibitor LY294002 administered in vivo 1 h before or immediately after ischemia or up to 120 h later significantly reverses preconditioning-induced protection, indicating a requirement for sustained PI3K signaling in ischemic tolerance. These findings implicate PI3K/Akt signaling in maintenance of the integrity of the mitochondrial outer membrane.

Ischemic insults promote epigenetic reprogramming of mu opioid receptor expression in hippocampal neurons

Transient global ischemia is a neuronal insult that induces delayed, selective death of hippocampal CA1 pyramidal neurons. A mechanism underlying ischemia-induced cell death is activation of the gene silencing transcription factor REST (repressor element-1 silencing transcription factor)/NRSF (neuron-restrictive silencing factor) and REST-dependent suppression of the AMPA receptor subunit GluR2 in CA1 neurons destined to die. Here we show that REST regulates an additional gene target, OPRM1 (mu opioid receptor 1 or MOR-1). MORs are abundantly expressed by basket cells and other inhibitory interneurons of CA1. Global ischemia induces a marked decrease in MOR-1 mRNA and protein expression that is specific to the selectively vulnerable area CA1, as assessed by quantitative real-time RT-PCR, Western blotting, and ChIP. We further show that OPRM1 gene silencing is REST-dependent and occurs via epigenetic modifications. Ischemia promotes deacetylation of core histone proteins H3 and H4 and dimethylation of histone H3 at lysine-9 (H3-K9) over the MOR-1 promoter, an signature of epigenetic gene silencing. Acute knockdown of MOR-1 gene expression by administration of antisense oligodeoxynucleotides to hippocampal slices in vitro or injection of the MOR antagonist naloxone to rats in vivo affords protection against ischemia-induced death of CA1 pyramidal neurons. These findings implicate MORs in ischemia-induced death of CA1 pyramidal neurons and document epigenetic remodeling of expression of OPRM1 in CA1 inhibitory interneurons.

New Goals in Ischemic Stroke Therapy: The Experimental Approach – Harmonizing Science with Practice

Undeniable advances have been made in clinical and experimental investigation into the pathophysiology, diagnosis, and treatment of cerebral ischemia. However, with the exception of intravenous thrombolysis and some neuroprotectors, such as citicoline, the majority of the drugs successfully tested in experimental studies have failed in clinical trials. Valuable lessons for the improvement of research methodology and appropriate coordination of experimental and clinical research can be learnt from the analysis of discrepancies between the laboratory and clinic, which will allow us to increase the power and cost-effectiveness of the studies. In addition, this progress has opened the way for the investigation of very promising new therapeutic strategies, such as combined pharmacological and mechanical thrombolysis, thrombolysis and neuroprotection, or the combination of various neuroprotectors, antiapoptotic therapies, and neurorestoration therapies, such as stem cell transplants.

Blockade of calcium-permeable AMPA receptors protects hippocampal neurons against global ischemia-induced death

Transient global or forebrain ischemia induced experimentally in animals can cause selective, delayed neuronal death of hippocampal CA1 pyramidal neurons. A striking feature is a delayed rise in intracellular free Zn(2+) in CA1 neurons just before the onset of histologically detectable cell death. Here we show that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) at Schaffer collateral to CA1 synapses in postischemic hippocampus exhibit properties of Ca(2+)/Zn(2+)-permeable, Glu receptor 2 (GluR2)-lacking AMPARs before the rise in Zn(2+) and cell death. At 42 h after ischemia, AMPA excitatory postsynaptic currents exhibited pronounced inward rectification and marked sensitivity to 1-naphthyl acetyl spermine (Naspm), a selective channel blocker of GluR2-lacking AMPARs. In control hippocampus, AMPA excitatory postsynaptic currents were electrically linear and relatively insensitive to Naspm. Naspm injected intrahippocampally at 9-40 h after insult greatly reduced the late rise in intracellular free Zn(2+) in postischemic CA1 neurons and afforded partial protection against ischemia-induced cell death. These results implicate GluR2-lacking AMPA receptors in the ischemia-induced rise in free Zn(2+) and death of CA1 neurons, although a direct action at the time of the rise in Zn(2+) is unproven. This receptor subtype appears to be an important therapeutic target for intervention in ischemia-induced neuronal death in humans.