Astrocytes exert a pro-apoptotic effect on neurons in postnatal hippocampal cultures

Early life phenobarbital exposure dysregulates the hippocampal transcriptome

Introduction: Phenobarbital (PB) and levetiracetam (LEV) are the first-line therapies for neonates with diagnosed seizures, however, a growing body of evidence shows that these drugs given during critical developmental windows trigger lasting molecular changes in the brain. While the targets and mechanism of action of these drugs are well understood-what is not known is how these drugs alter the transcriptomic landscape, and therefore molecular profile/gene expression during these critical windows of neurodevelopment. PB is associated with a range of neurotoxic effects in developing animals, from cell death to altered synaptic development to lasting behavioral impairment. LEV does not produce these effects. Methods: Here we evaluated the effects of PB and Lev on the hippocampal transcriptome by RNA sequencing. Neonatal rat pups were given a single dose of PB, Lev or vehicle and sacrificed 72 h later-at time at which drug is expected to be cleared. Results: We found PB induces broad changes in the transcriptomic profile (124 differentially expressed transcripts), as compared to relatively small changes in LEV-treated animals (15 transcripts). PB exposure decreased GABAergic and oligodendrocyte markers pvalb and opalin, and increased the marker of activated microglia, cd68 and the astrocyte- associated gene vegfa. These data are consistent with the existing literature showing developmental neurotoxicity associated with PB, but not LEV. Discussion: The widespread change in gene expression after PB, which affected transcripts reflective of multiple cell types, may provide a link between acute drug administration and lasting drug toxicity.

Astrocytes and microglia in the coordination of CNS development and homeostasis

Glia have emerged as important architects of central nervous system (CNS) development and maintenance. While traditionally glial contributions to CNS development and maintenance have been studied independently, there is growing evidence that either suggests or documents that glia may act in coordinated manners to effect developmental patterning and homeostatic functions in the CNS. In this review, we focus on astrocytes, the most abundant glia in the CNS, and microglia, the earliest glia to colonize the CNS highlighting research that documents either suggestive or established coordinated actions by these glial cells in various CNS processes including cell and/or debris clearance, neuronal survival and morphogenesis, synaptic maturation, and circuit function, angio-/vasculogenesis, myelination, and neurotransmission. Some molecular mechanisms underlying these processes that have been identified are also described. Throughout, we categorize the available evidence as either suggestive or established interactions between microglia and astrocytes in the regulation of the respective process and raise possible avenues for further research. We conclude indicating that a better understanding of coordinated astrocyte-microglial interactions in the developing and mature brain holds promise for developing effective therapies for brain pathologies where these processes are perturbed.

Ambient but not local lactate underlies neuronal tolerance to prolonged glucose deprivation

Neurons require a nearly constant supply of ATP. Glucose is the predominant source of brain ATP, but the direct effects of prolonged glucose deprivation on neuronal viability and function remain unclear. In sparse rat hippocampal microcultures, neurons were surprisingly resilient to 16 h glucose removal in the absence of secondary excitotoxicity. Neuronal survival and synaptic transmission were unaffected by prolonged removal of exogenous glucose. Inhibition of lactate transport decreased microculture neuronal survival during concurrent glucose deprivation, suggesting that endogenously released lactate is important for tolerance to glucose deprivation. Tandem depolarization and glucose deprivation also reduced neuronal survival, and trace glucose concentrations afforded neuroprotection. Mass cultures, in contrast to microcultures, were insensitive to depolarizing glucose deprivation, a difference attributable to increased extracellular lactate levels. Removal of local astrocyte support did not reduce survival in response to glucose deprivation or alter evoked excitatory transmission, suggesting that on-demand, local lactate shuttling is not necessary for neuronal tolerance to prolonged glucose removal. Taken together, these data suggest that endogenously produced lactate available globally in the extracellular milieu sustains neurons in the absence of glucose. A better understanding of resilience mechanisms in reduced preparations could lead to therapeutic strategies aimed to bolster these mechanisms in vulnerable neuronal populations.

The Indispensable Roles of Microglia and Astrocytes during Brain Development

Glia are essential for brain functioning during development and in the adult brain. Here, we discuss the various roles of both microglia and astrocytes, and their interactions during brain development. Although both cells are fundamentally different in origin and function, they often affect the same developmental processes such as neuro-/gliogenesis, angiogenesis, axonal outgrowth, synaptogenesis and synaptic pruning. Due to their important instructive roles in these processes, dysfunction of microglia or astrocytes during brain development could contribute to neurodevelopmental disorders and potentially even late-onset neuropathology. A better understanding of the origin, differentiation process and developmental functions of microglia and astrocytes will help to fully appreciate their role both in the developing as well as in the adult brain, in health and disease.

HIV-1 Tat Promotes Lysosomal Exocytosis in Astrocytes and Contributes to Astrocyte-mediated Tat Neurotoxicity

Tat interaction with astrocytes has been shown to be important for Tat neurotoxicity and HIV/neuroAIDS. We have recently shown that Tat expression leads to increased glial fibrillary acidic protein (GFAP) expression and aggregation and activation of unfolded protein response/endoplasmic reticulum (ER) stress in astrocytes and causes neurotoxicity. However, the exact molecular mechanism of astrocyte-mediated Tat neurotoxicity is not defined. In this study, we showed that neurotoxic factors other than Tat protein itself were present in the supernatant of Tat-expressing astrocytes. Two-dimensional gel electrophoresis and mass spectrometry revealed significantly elevated lysosomal hydrolytic enzymes and plasma membrane-associated proteins in the supernatant of Tat-expressing astrocytes. We confirmed that Tat expression and infection of pseudotyped HIV.GFP led to increased lysosomal exocytosis from mouse astrocytes and human astrocytes. We found that Tat-induced lysosomal exocytosis was tightly coupled to astrocyte-mediated Tat neurotoxicity. In addition, we demonstrated that Tat-induced lysosomal exocytosis was astrocyte-specific and required GFAP expression and was mediated by ER stress. Taken together, these results show for the first time that Tat promotes lysosomal exocytosis in astrocytes and causes neurotoxicity through GFAP activation and ER stress induction in astrocytes and suggest a common cascade through which aberrant astrocytosis/GFAP up-regulation potentiates neurotoxicity and contributes to neurodegenerative diseases.

Optical control after transfection of channelrhodopsin-2 recombinant adenovirus in visual cortical cells

Channelrhodopsin-2 ectopically expressed in the retina can recover the response to blue light in genetically blind mice and rats, but is unable to restore visual function due to optic nerve or optic tract lesions. Long Evans rats at postnatal day 1 were used for primary culture of visual cortical cells, and 24 hours later, cells were transfected with recombinant adenovirus carrying channelrhodopsin-2 and green fluorescent protein genes. After 2-4 days of transfection, green fluorescence was visible in the cultured cells. Cells were stimulated with blue light (470 nm), and light-induced action potentials were recorded in patch-clamp experiments. Our findings indicate that channelrhodopsin-2-recombinant adenovirus transfection of primary cultured visual cortical cells can control the production of action potentials via blue light stimulation.

Excitotoxicity triggered by Neurobasal culture medium

Neurobasal defined culture medium has been optimized for survival of rat embryonic hippocampal neurons and is now widely used for many types of primary neuronal cell culture. Therefore, we were surprised that routine medium exchange with serum- and supplement-free Neurobasal killed as many as 50% of postnatal hippocampal neurons after a 4 h exposure at day in vitro 12–15. Minimal Essential Medium (MEM), in contrast, produced no significant toxicity. Detectable Neurobasal-induced neuronal death occurred with as little as 5 min exposure, measured 24 h later. D-2-Amino-5-phosphonovalerate (D-APV) completely prevented Neurobasal toxicity, implicating direct or indirect N-methyl-D-aspartate (NMDA) receptor-mediated neuronal excitotoxicity. Whole-cell recordings revealed that Neurobasal but not MEM directly activated D-APV-sensitive currents similar in amplitude to those gated by 1 µM glutamate. We hypothesized that L-cysteine likely mediates the excitotoxic effects of Neurobasal incubation. Although the original published formulation of Neurobasal contained only 10 µM L-cysteine, commercial recipes contain 260 µM, a concentration in the range reported to activate NMDA receptors. Consistent with our hypothesis, 260 µM L-cysteine in bicarbonate-buffered saline gated NMDA receptor currents and produced toxicity equivalent to Neurobasal. Although NMDA receptor-mediated depolarization and Ca²⁺ influx may support survival of young neurons, NMDA receptor agonist effects on development and survival should be considered when employing Neurobasal culture medium.

Presynaptic silencing is an endogenous neuroprotectant during excitotoxic insults

Glutamate release is a root cause of acute and delayed neuronal damage in response to hypoxic/ischemic insults. Nevertheless, therapeutics that target the postsynaptic compartment have been disappointing clinically. Here we explored whether presynaptic silencing (muting) of glutamatergic terminals is sufficient to reduce excitotoxic damage resulting from hypoxia and oxygen/glucose deprivation. Our evidence suggests that strong depolarization, previously shown to mute glutamate synapses, protects neurons by a presynaptic mechanism that is sensitive to inhibition of the proteasome. Postsynaptic Ca2+ rises in response to glutamate application and toxicity in response to exogenous glutamate treatment were unaffected by depolarization preconditioning. These features strongly suggest that reduced glutamate release explains preconditioning protection. We addressed whether hypoxic depolarization itself induces presynaptic silencing, thereby participating in the damage threshold for hypoxic insult. Indeed, we found that the hypoxic insult increased the percentage of mute glutamate synapses in a proteasome-dependent manner. Furthermore, proteasome inhibition exacerbated neuronal loss to mild hypoxia and prevented hypoxia-induced muting. In total our results suggest that presynaptic silencing is an endogenous neuroprotective mechanism that could be exploited to reduce damage from insults involving excess synaptic glutamate release.

Magnesium induces neuronal apoptosis by suppressing excitability

In clinical obstetrics, magnesium sulfate (MgSO(4)) use is widespread, but effects on brain development are unknown. Many agents that depress neuronal excitability increase developmental neuroapoptosis. In this study, we used dissociated cultures of rodent hippocampus to examine the effects of Mg(++) on excitability and survival. Mg(++)-induced caspase-3-associated cell loss at clinically relevant concentrations. Whole-cell patch-clamp techniques measured Mg(++) effects on action potential threshold, action potential peak amplitude, spike number and changes in resting membrane potential. Mg(++) depolarized action potential threshold, presumably from surface charge screening effects on voltage-gated sodium channels. Mg(++) also decreased the number of action potentials in response to fixed current injection without affecting action potential peak amplitude. Surprisingly, Mg(++) also depolarized neuronal resting potential in a concentration-dependent manner with a +5.2 mV shift at 10 mM. Voltage ramps suggested that Mg(++) blocked a potassium conductance contributing to the resting potential. In spite of this depolarizing effect of Mg(++), the net inhibitory effect of Mg(++) nearly completely silenced neuronal network activity measured with multielectrode array recordings. We conclude that although Mg(++) has complex effects on cellular excitability, the overall inhibitory influence of Mg(++) decreases neuronal survival. Taken together with recent in vivo evidence, our results suggest that caution may be warranted in the use of Mg(++) in clinical obstetrics and neonatology.

Dimethyl sulfoxide (DMSO) produces widespread apoptosis in the developing central nervous system

Dimethyl sulfoxide (DMSO) is a solvent that is routinely used as a cryopreservative in allogous bone marrow and organ transplantation. We exposed C57Bl/6 mice of varying postnatal ages (P0-P30) to DMSO in order to study whether DMSO could produce apoptotic degeneration in the developing CNS. DMSO produced widespread apoptosis in the developing mouse brain at all ages tested. Damage was greatest at P7. Significant elevations above the background rate of apoptosis occurred at the lowest dose tested, 0.3 ml/kg. In an in vitro rat hippocampal culture preparation, DMSO produced neuronal loss at concentrations of 0.5% and 1.0%. The ability of DMSO to damage neurons in dissociated cultures indicates that the toxicity likely results from a direct cellular effect. Because children, who undergo bone marrow transplantation, are routinely exposed to DMSO at doses higher than 0.3 ml/kg, there is concern that DMSO might be producing similar damage in human children.

Bone marrow stromal cells induce BMP2/4 production in oxygen-glucose-deprived astrocytes, which promotes an astrocytic phenotype in adult subventricular progenitor cells

Bone morphogenetic proteins (BMPs) affect cell proliferation and differentiation. Astrocytes in ischemic brain are highly responsive to bone marrow stromal cell (BMSC) treatment. We investigated the effects of BMSCs on astrocytes cultured under oxygen- and glucose-deprived conditions, which in part simulate in vivo stroke conditions, to test the hypothesis that BMSCs alter astrocytic expression of BMPs which may contribute to neurological functional recovery of stroke. Quantitative real-time RT-PCR showed that the expression of BMP2/4 mRNAs decreased within ischemic astrocytes, In contrast, BMP2/4 mRNA was significantly increased after cocultured with BMSCs. Western blotting also confirmed this increase at the protein level in the medium of ischemic astrocytes after coculture with BMSCs. As a source of neural stem and progenitor cells, cultured subventricular zone (SVZ) neurospheres exposed to medium obtained from ischemic astrocytes cocultured with BMSCs were significantly enriched in cells expressing the astrocytic marker glial fibrillary acidic protein (GFAP), but not at the expense of beta-III-tubulin-positive SVZ neuroblasts. The expression of BMP2/4 subsequently increased the phosphorylation of downstream effector Smad1 and the expression of notch signal pathway-induced protein Hes1 in cultured SVZ neurospheres. BMP antagonist Noggin blocked the elevation of phosphorylated Smad1 and the expression of Hes1 as well as reducing the percentage of astrocytic SVZ progenitor cells. Our results indicate that BMSCs increase BMP2/4 expression in ischemic astrocytes. These changes enhance subventricular progenitor cell gliogenesis by activating relevant signaling pathways. BMSC-stimulated signaling of endogenous astrocytes may alter the ischemic environment, promoting remodeling of brain and hence, improve functional recovery after stroke.