Catherine Colin

Microglia promote the death of developing Purkinje cells

The loss of neuronal cells, a prominent event in the development of the nervous system, involves regulated triggering of programmed cell death, followed by efficient removal of cell corpses. Professional phagocytes, such as microglia, contribute to the elimination of dead cells. Here we provide evidence that, in addition to their phagocytic activity, microglia promote the death of developing neurons engaged in synaptogenesis. In the developing mouse cerebellum, Purkinje cells die, and 60% of these neurons that already expressed activated caspase-3 were engulfed or contacted by spreading processes emitted by microglial cells. Apoptosis of Purkinje cells in cerebellar slices was strongly reduced by selective elimination of microglia. Superoxide ions produced by microglial respiratory bursts played a major role in this Purkinje cell death. Our study illustrates a mammalian form of engulfment-promoted cell death that links the execution of neuron death to the scavenging of dead cells.

Neurotoxic Activation of Microglia Is Promoted by a Nox1-Dependent NADPH Oxidase

Reactive oxygen species (ROS) modulate intracellular signaling but are also responsible for neuronal damage in pathological states. Microglia, the resident CNS macrophages, are prominent sources of ROS through expression of the phagocyte oxidase which catalytic subunit Nox2 generates superoxide ion (O2·−). Here we show that microglia also express Nox1 and other components of nonphagocyte NADPH oxidases, including p22phox, NOXO1, NOXA1, and Rac1/2. The subcellular distribution and functions of Nox1 were determined by blocking Nox activity with diphenylene iodonium or apocynin, and by silencing the Nox1 gene in microglia purified from wild-type (WT) or Nox2-KO mice. [Nox1-p22phox] dimers localized in intracellular compartments are recruited to phagosome membranes during microglial phagocytosis of zymosan, and Nox1 produces O2·− in zymosan-loaded phagosomes. In microglia activated with lipopolysaccharide (LPS), Nox1 produces O2·−, which enhances cell expression of inducible nitric oxide synthase and secretion of interleukin-1β. Comparisons of microglia purified from WT, Nox2-KO, or Nox1-KO mice indicate that both Nox1 and Nox2 are required to optimize microglial production of nitric oxide. By injecting LPS in the striatum of WT and Nox1-KO mice, we show that Nox1 also enhances microglial production of cytotoxic nitrite species and promotes loss of presynaptic proteins in striatal neurons. These results demonstrate the functional expression of Nox1 in resident CNS phagocytes, which can promote production of neurotoxic compounds during neuroinflammation. Our study also shows that Nox1- and Nox2-dependent oxidases play distinct roles in microglial activation and that Nox1 is a possible target for the treatment of neuroinflammatory states.

System Xc− and Apolipoprotein E Expressed by Microglia Have Opposite Effects on the Neurotoxicity of Amyloid-β Peptide 1–40

Because senile plaques in Alzheimers disease (AD) contain reactive microglia in addition to potentially neurotoxic aggregates of amyloid-β (Aβ), we examined the influence of microglia on the viability of rodent neurons in culture exposed to aggregated Aβ 1–40. Microglia enhanced the toxicity of Aβ by releasing glutamate through the cystine-glutamate antiporter system Xc−. This may be relevant to Aβ toxicity in AD, because the system Xc−-specific xCT gene is expressed not only in cultured microglia but also in reactive microglia within or surrounding amyloid plaques in transgenic mice expressing mutant human amyloid precursor protein or in wild-type mice injected with Aβ. Inhibition of NMDA receptors or system Xc− prevented the microglia-enhanced neurotoxicity of Aβ but also unmasked a neuroprotective effect of microglia mediated by microglial secretion of apolipoprotein E (apoE) in the culture medium. Immunodepletion of apoE or targeted inactivation of the apoE gene in microglia abrogated neuroprotection by microglial conditioned medium, whereas supplementation by human apoE isoforms restored protection, which was potentiated by the presence of microglia-derived cofactors. These results suggest that inhibition of microglial system Xc− might be of therapeutic value in the treatment of AD. Its inhibition not only prevents glutamate excitotoxicity but also facilitates neuroprotection by apoE.

Specific pattern of nitric oxide synthase expression in glial cells after hippocampal injury.

In the central nervous system (CNS), nitric oxide (NO) is thought to be involved in a variety of functions including synaptic plasticity, long term potentiation, and neurotoxicity. The aim of the present study was to investigate the expression of nitric oxide synthase (NOS) in the mouse CNS, following surgical injury to the hippocampus. NOS expression was assessed by histochemical detection of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH‐diaphorase) activity and immunohistochemistry of the inducible NOS (iNOS). Two days after injury to the CA1 hippocampal field, NADPH‐diaphorase activity was detected in pyramidal and granular neurons and also in glial cells in the hippocampus, in contrast to the non‐injured one where NADPH‐diaphorase staining was observed only in a few interneurons. NADPH‐diaphorase histochemistry combined with immunolabelling for GFAP and F4/80 demonstrated that these glial cells were astrocytes and microglia. This pattern of NOS expression is induced specifically after a hippocampal injury since lesion to the prefrontal or cerebellar cortex leads to NOS activity only in monocytes/macrophages like cells. Despite the large expression of NOS detected by NADPH‐diaphorase histochemistry after lesioning the hippocampus, immunostaining for iNOS was confined to microglia. The fact that induction of high levels of NOS activity are detected in glial cells after a lesion to the hippocampus could be accounted for by the sensitivity of this structure to a high release of glutamate. GLIA 22:329–337, 1998.

The NADPH oxidase Nox2 regulates VEGFR1/CSF-1R-mediated microglial chemotaxis and promotes early postnatal infiltration of phagocytes in the subventricular zone of the mouse cerebral cortex

The phagocyte NADPH oxidase Nox2 generates superoxide ions implicated in the elimination of microorganisms and the redox control of inflammatory signaling. However, the role of Nox2 in phagocyte functions unrelated to immunity or pathologies is unknown. During development, oriented cell migrations insure the timely recruitment and function of phagocytes in developing tissues. Here, we have addressed the role of Nox2 in the directional migration of microglial cells during development. We show that microglial Nox2 regulates the chemotaxis of purified microglia mediated by the colony stimulating factor‐1 receptor (CSF‐1R) and the vascular endothelial growth factor receptor‐1 (VEGFR1). Stimulation of these receptors triggers activation of Nox2 at the leading edge of polarized cells. In the early postnatal stages of mouse brain development, Nox2 is activated in macrophages / microglial cells in the lateral ventricle or the adjacent subventricular zone (SVZ). Fluorescent microglia injected into the lateral ventricle infiltrate the dorso‐caudal SVZ through a mechanism that is blocked by pretreatment of the injected cells with an irreversible Nox inhibitor. Infiltration of endogenous microglia into the caudal SVZ of the cerebral cortex is prevented by (1) Nox2 gene deficiency, (2) treatment with a Nox2 inhibitor (apocynin), and (3) invalidation of the VEGFR1 kinase. We conclude that phagocytes move out of the lateral ventricle soon after birth and infiltrate the cortical SVZ through a mechanism requiring microglial Nox2 and VEGFR1 activation. Nox2 therefore modulates the migration of microglia and their development.

New insights into the role of thyroid hormone in the CNS: the microglial track.

Mononuclear phagocytes called microglia, account for a significant part of the cells residing in the CNS; recent evidence indicates that thyroid hormone plays an important role in their development. Microglial cell processes bearing markers specific for mononuclear phagocytes are observed throughout the CNS. They form a network that is responsive to even mild alterations of the tissue integrity. Injury to CNS parenchyma promotes rapid microglial activation characterized by morphological changes, increased expression of macrophage markers, and upregulation of functional capacities which vary according to the type of lesion, but typically include release of inflammatory mediators and phagocytosis of degenerating cells. Activated microglia appear in the course of various neurological diseases, and have also been reported in psychiatric illness such as schizophrenia. The activation of microglial cells is thought to have major pathophysiological consequences in infectious or neurodegenerative disorders with progressive decline of cognitive function, such as HIV dementia or Alzheimer’s disease. Under normal conditions, the function of microglia is most obvious during development, where they eliminate apoptotic cell bodies generated by the regressive events of neurogenesis. However, the microglial cells might also play a role in the survival of developing neurons or the growth of neuronal processes. Characterization of the physiologic factors which regulate the establishment of the microglial network is a challenging issue with respect to both normal development and pathological remodeling of the CNS. Recent evidence has highlighted the role of thyroid hormone (TH) as an important promoter of microglial growth and morphological differentiation. The development of microglia involves a progressive increase in the number of these cells that results from both infiltration into the CNS of mesodermal microglial precursors generated in hemopoietic organs and proliferation of microglial cells within the developing CNS. Concomitantly, the microglial cells undergo maturation which is characterized morphologically by the growth of tortuous ramified processes. However, degeneration of developing microglial cells limits the expansion of this cell population. In the forebrain of developing rats, the increase in the number of microglial cells and the growth of their processes are

Remyelination by Resident Oligodendrocyte Precursor Cells in a Xenopus laevis Inducible Model of Demyelination.

We have generated a Xenopus laevis transgenic line, MBP-GFP-NTR, allowing conditional ablation of myelin-forming oligodendrocytes. In this transgenic line the transgene is driven by the proximal portion of the myelin basic protein regulatory sequence, specific to mature oligodendrocytes. The transgene protein is formed by the green fluorescent protein reporter fused to the Escherichia coli nitroreductase (NTR) selection enzyme. The NTR enzyme converts the innocuous prodrug metronidazole (MTZ) to a cytotoxin. Ablation of oligodendrocytes by MTZ treatment of the tadpole induced demyelination, and here we show that myelin debris are subsequently eliminated by microglial cells. After cessation of MTZ treatment, remyelination proceeded spontaneously. We questioned the origin of remyelinating cells. Our data suggest that Sox10+ oligodendrocyte precursor cells (OPCs), which are already present in the optic nerve prior to the experimentally induced demyelination, are responsible for remyelination, and this required only minimal (if any) cell division of OPCs.

Les faibles doses de radiations : vers une nouvelle lecture de l’évaluation du risque ?

From Hiroshima bomb explosion data, the risk of radiation-induced cancer is significant from 100xa0mSv for a population considered as uniform and radioresistant. However, the recent radiobiological data bring some new elements that highlight some features that were not taken into account: the individual factor, the dose rate and the repeated dose effect. The objective evaluation of the cancer risk due to doses lower than 100xa0mSv is conditioned by high levels of measurability and statistical significance. However, it appears that methodological rigor is not systematically applied in all the papers. Furthermore, unclear communication in press often leads to some announcement effects, which does not improve the readability of the issue. This papers aims to better understand the complexity of the low-dose-specific phenomena as a whole, by confronting the recent biological data with epidemiological data.

Fine structure of astroglial integration into host brain following xenografting

Previous investigations showed that fragments of fetal rabbit brain transplanted into striatum of neonatal shiverer mouse give rise to cells that migrate through host tissue and differentiate into astroglia and oligodendroglia within 2 weeks. We studied the integration of transplanted astroglia at the ultrastructurul level using pre-cmbedding labeling with a monoclonal antibody which recognizes an epitope associated with rabbit but not mouse glial fibrillary acidic protein. The morphology of early migrating donor cells docs not distinguish them from cells arising in host germinal matrix. Once the cells complete their migration they integrate into host brain in a structurally normal manner. Transplanted astroglia form perivascular foot plates with host capillaries. They also send extensive processes into the neuropil where intimate contacts with neurons and synaptic structures are formed. Oligodendroglia send processes to nearby axons where they form normal-appearing myelin. During the rejection process, which may begin at 4 weeks, donor astroglia show evidence of reaction with increased intermediate filament content. Donor cells are attacked by leukocytes, including eosinophils, and subsequently degenerate. We conclude that crossspecies transplantation of glial cells can result in entirely normal structural integration into host brain.

DNA breaks induced by iodine-containing contrast medium in radiodiagnostics: a problem of tungsten?

Iodine-containing contrast media (ICM) are extensively used to improve image quality and information content in x-ray-based examinations, particularly in computed tomography (CT). In parallel, there is increasing evidence that the use of ICM during CT sessions is associated with deoxyribonucleic acid (DNA) breaks that may influence the estimation of the risks linked to x-ray exposure. Why has iodine been preferred to any other heavy elements to enhance contrast in radiodiagnostics? How to understand such DNA breaks effect? We searched for the answers in the early times of x-ray medical use. It appeared that the maximal ratio between the relative iodine and water mass energy absorption coefficients is reached in the range of 40–60xa0keV, which defines the energy range in which the dose is preferentially absorbed by ICM. This range does not correspond to the K-edge of iodine but to that of tungsten, the major component of the x-ray tube anode of CT scanners. At such energy, radiolysis of the ICM produces sodium or potassium iodide that prevents a normal DNA breaks repair and influences the individual response to x-ray low-dose. Both contrast enhancement and DNA breaks effect may therefore be caused by tungsten of the anodes of x-ray tubes.