Imbalances in neurotrophins or their high-affinity Trk receptors have long been reported in neurodegenerative diseases. However, a molecular link between these gene products and neuronal cell death has not been established. In the trisomy 16 (Ts16) mouse there is increased apoptosis in the cortex, and hippocampal neurons undergo accelerated cell death that cannot be rescued by administration of brain-derived neurotrophic factor (BDNF). Ts16 neurons have normal levels of the TrkB tyrosine kinase receptor but an upregulation of the TrkB.T1 truncated receptor isoform. Here we show that restoration of the physiological level of the TrkB.T1 receptor by gene targeting rescues Ts16 cortical cell and hippocampal neuronal death. Moreover, it corrects resting Ca2+ levels and restores BDNF-induced intracellular signaling mediated by full-length TrkB in Ts16 hippocampal neurons. These data provide a direct link between neuronal cell death and abnormalities in Trk neurotrophin receptor levels.

Research on ischemic brain injury has established a central role of mitochondria in neuron death. Astrocytes are also damaged by ischemia, although the participation of mitochondria in their injury is ill defined. As astrocytes are responsible for neuronal metabolic and trophic support, astrocyte dysfunction will compromise postischemic neuronal survival. Ischemic alterations to astrocyte energy metabolism and the uptake and metabolism of the excitatory amino acid transmitter glutamate may be particularly important. Despite the significance of ischemic astrocyte injury, little is known of the mechanisms responsible for astrocyte death and dysfunction. This review focuses on differences between astrocyte and neuronal metabolism and mitochondrial function, and on neuronal-glial interactions. The potential for astrocyte mitochondria to serve as targets of neuroprotective interventions is also discussed.

Hypoxic/ischemic (HI) brain injury in newborn full-term and premature infants is a common and pervasive source of life time disabilities in cognitive and locomotor function. In the adult, HI induces glutamate release and excitotoxic cell death dependent on NMDA receptor activation. In animal models of the premature human infant, glutamate is also released following HI, but neurons are largely insensitive to NMDA or AMPA/kainic acid (KA) receptor-mediated damage. Using primary cultured hippocampal neurons we have determined that glutamate increases intracellular calcium much more than kainic acid. Moreover, glutamate induces cell death by activating Type I metabotropic glutamate receptors (mGluRs). Pretreatment of neurons with the gonadal steroid estradiol reduces the level of the Type I metabotropic glutamate receptors and completely prevents cell death, suggesting a novel therapeutic approach to excitotoxic brain damage in the neonate.

The neurotrophin brain-derived neurotrophic factor (BDNF), via activation of its receptor, tyrosine receptor kinase B (trkB), regulates a wide variety of cellular processes in the nervous system, including neuron survival and synaptic plasticity. Although the expression of BDNF is known to be Ca 2؉ -dependent, the regulation of trkB expression has not been extensively studied. Here we report that depolarization of cultured mouse cortical neurons increased the expression of the full-length, catalytically active isoform of trkB without affecting expression of the truncated isoform. This increase in protein expression was accompanied by increased levels of transcripts encoding full-length, but not truncated, trkB. Depolarization also regulated transcription of the gene, TRKB, via entry of Ca 2؉ through voltage-gated Ca 2؉ channels and subsequent activation of Ca 2؉ -responsive elements in the two TRKB promoters. Using transient transfection of neurons with TRKB promoterluciferase constructs, we found that Ca 2؉ inhibited the upstream promoter P1 but activated the downstream promoter P2. Ca 2؉ -dependent stimulation of TRKB expression requires two adjacent, non-identical CRE sites located within P2. The coordinated regulation of BDNF and trkB by Ca 2؉ may play a role in activity-dependent survival and synaptic plasticity by enhancing BDNF signaling in electrically active neurons.The neurotrophin, brain-derived neurotrophic factor (BDNF), 1 mediates numerous functions in both the developing and mature nervous systems, including the survival of postmitotic neurons, axon growth and guidance, and synaptic plasticity (1). These effects of BDNF are mediated by the tyrosine receptor kinase, trkB. Binding of BDNF to trkB initiates dimerization and trans-autophosphorylation of tyrosine residues in the intracellular domain of trkB (2). These phosphotyrosine residues act as docking sites for effector proteins that activate downstream signaling pathways, leading to the activation of protein kinase cascades, Ca 2ϩ mobilization, and gene expression, which orchestrate the cellular responses to BDNF (3). Excitatory synaptic input and the resulting elevation in intracellular [Ca 2ϩ ] have been shown to increase the synthesis and release of BDNF (4 -9). This BDNF activates trkB receptors in the same or neighboring cells to promote their survival and may also enhance synaptic plasticity (1, 10). Although trkB levels change during development and exhibit cell-specific expression patterns (11-13), very little is known about the mechanisms that regulate TRKB expression.At least four isoforms of trkB are produced by alternative splicing of the primary transcripts of the TRKB gene (14 -16). Of these, only the full-length isoform, which contains an intracellular tyrosine kinase domain, is known to be capable of mediating BDNF signaling. Three truncated isoforms (T1, T2, and T shc ), which lack the intracellular kinase domain but possess the same extracellular BDNF binding domain as fulllength receptors, can also be generated by alternativ...

Oxidative stress is a mediator of cell death following cerebral ischemia/reperfusion and heme toxicity which can be an important pathogenic factor in acute brain injury. Induced expression of Phase II detoxification enzymes through activation of the anti-oxidant response element (ARE)/ Nrf2 pathway has emerged as a promising approach for neuroprotection. Little is known, however, about the neuroprotective potential of this strategy against injury immature brain cells. In this study, we tested the hypothesis that sulforaphane (SFP), a naturally occurring isothiocyanate that is also a known activator of the ARE/Nrf2 antioxidant pathway, can protect immature neurons from oxidative stress-induced death. The hypothesis was tested with primary mouse hippocampal neurons exposed to either O 2 and glucose deprivation (OGD) or hemin. Treatment of immature neurons with SFP immediately after the OGD during reoxygenation was effective at protecting immature neurons from delayed cell death. Exposure of immature hippocampal neurons to hemin induced significant cell death and both pre-and co-treatment with SFP were remarkably effective at blocking cytotoxicity. RT-PCR analysis indicated that several Nrf2-dependent cytoprotective genes, including NAD(P)H quinone oxidoreductase 1 (NQO1), heme oxygenase 1 (HO1) and glutamate-cysteine ligase modifier subunit (GCLM) that is involved in glutathione biosynthesis, were upregulated following SFP treatment both in control neurons and following exposure to OGD and hemin. These results indicate that SFP activates the ARE/Nrf2 pathway of antioxidant defense and protects immature neurons from death caused by stress paradigms relevant to those associated with ischemic and traumatic injury to the immature brain.