Early changes of macrophage-like immunoreactivity in the rat inferior olive after intraperitoneal 3-acetylpyridine injection

Tandem insults of prenatal ischemia plus postnatal raised intrathoracic pressure in a novel rat model of encephalopathy of prematurity

OBJECT

Encephalopathy of prematurity (EP) is common in preterm, low birth weight infants who require postnatal mechanical ventilation. The worst types of EP are the hemorrhagic forms, including choroid plexus, germinal matrix, periventricular, and intraventricular hemorrhages. Survivors exhibit life-long cognitive, behavioral, and motor abnormalities. Available preclinical models do not fully recapitulate the salient features of hemorrhagic EP encountered in humans. In this study, the authors evaluated a novel model using rats that featured tandem insults of transient prenatal intrauterine ischemia (IUI) plus transient postnatal raised intrathoracic pressure (RIP).

METHODS

Timed-pregnant Wistar rats were anesthetized and underwent laparotomy on embryonic Day 19. Intrauterine ischemia was induced by clamping the uterine and ovarian vasculature for 20 minutes. Natural birth occurred on embryonic Day 22. Six hours after birth, the pups were subjected to an episode of RIP, induced by injecting glycerol (50%, 13 μl/g intraperitoneally). Control groups included naive, sham surgery, and IUI alone. Pathological, histological, and behavioral analyses were performed on pups up to postnatal Day 52.

RESULTS

Compared with controls, pups subjected to IUI+RIP exhibited significant increases in postnatal mortality and hemorrhages in the choroid plexus, germinal matrix, and periventricular tissues as well as intraventricularly. On postnatal Days 35-52, they exhibited significant abnormalities involving complex vestibulomotor function and rapid spatial learning. On postnatal Day 52, the brain and body mass were significantly reduced.

CONCLUSIONS

Tandem insults of IUI plus postnatal RIP recapitulate many features of the hemorrhagic forms of EP found in humans, suggesting that these insults in combination may play important roles in pathogenesis.

Involvement of astroglial fibroblast growth factor-2 and microglia in the nigral 6-OHDA parkinsonism and a possible role of glucocorticoid hormone on the glial mediated local trophism and wound repair

We have observed in previous studies that 6-hydroxydopamine (6-OHDA)-induced lesions in the nigrostriatal dopamine (DA) system promote increases of the astroglial basic fibroblast growth factor (FGF-2, bFGF) synthesis in the ascending DA pathways, event that could be modified by adrenosteroid hormones. Here, we first evaluated the changes of microglial reactivity in relation to the FGF-2-mediated trophic responses in the lesioned nigrostriatal DA system. 6-OHDA was injected into the left side of the rat substantia nigra. The OX42 immunohistochemistry combined with stereology showed the time course of the microglial activation. The OX42 immunoreactivity (IR) was already increased in the pars compacta of the substantia nigra (SNc) and ventral tegmental area (VTA) 2 h after the 6-OHDA injection, peaked on day 7, and remained increased on the 14th day time-interval. In the neostriatum, OX42 immunoreactive (ir) microglial profiles increased at 24 h, peaked at 72 h, was still increased at 7 days but not 14 days after the 6-OHDA injection. Two-colour immunofluorescence analysis of the tyrosine hydroxylase (TH) and OX42 IRs revealed the presence of small patches of TH IR within the activated microglia. A decreased FGF-2 IR was seen in the cytoplasm of DA neurons of the SNc and VTA as soon as 2 h after 6-OHDA injection. The majority of the DA FGF-2 ir cells of these regions had disappeared 72 h after neurotoxin. The astroglial FGF-2 IR increased in the SNc and VTA, which peaked on day 7. Two-colour immunofluorescence and immunoperoxidase analyses of the FGF-2 and OX42 IRs revealed no FGF-2 IR within the reactive or resting microglia. Second, we have evaluated in a series of biochemical experiments whether adrenocortical manipulation can interfere with the nigral lesion and the state of local astroglial reaction, looking at the TH and GFAP levels respectively. Rats were adrenalectomized (ADX) and received a nigral 6-OHDA stereotaxical injection 2 days later and sacrificed up to 3 weeks after the DA lesion. Western blot analysis showed time-dependent decrease and elevation of TH and GFAP levels, respectively, in the lesioned versus contralateral midbrain sides, events potentiated by ADX and worsened by corticosterone replacement. ADX decreased the levels of FGF-2 protein (23 kDa isoform) in the lesioned side of the ventral midbrain compared contralaterally. The results indicate that reactive astroglia, but not reactive microglia, showed an increased FGF-2 IR in the process of DA cell degeneration induced by 6-OHDA. However, interactions between these glial cells may be relevant to the mechanisms which trigger the increased astroglial FGF-2 synthesis and thus may be related to the trophic state of DA neurons and the repair processes following DA lesion. The findings also gave further evidence that adrenocortical hormones may regulate astroglial-mediated trophic mechanisms and wound repair events in the lesioned DA system that may be relevant to the progression of Parkinson's disease.

Microglia in the cerebral wall of the human telencephalon at second trimester

We have recently begun to gain a clearer understanding of the phasing and patterns of colonization of the developing human brain by microglia. In this study we investigated the distribution, morphology and phenotype of microglia specifically within the wall of the human telencephalon from 12 to 24 gestational weeks (gw), a period that corresponds to the development of thalamocortical fibres passing through the transient subplate region of the developing cerebral wall. Sections from a total of 45 human fetal brains were immunoreacted to detect CD68 and MHC class II antigens and histochemically reacted with RCA-1 and tomato lectins. These markers were differentially expressed by anatomically discrete populations of microglia in the cerebral wall: two cell populations were noted during the initial phase of colonization (12-14 gw): (i) CD68++ RCA-1+ MHC II- amoeboid cells aligned within the subplate, and (ii) RCA-1++ CD68- MHC II- progenitors in the marginal layer and lower cortical plate that progressively ramified within the subplate, without seemingly passing through an 'amoeboid' state. At this stage microglia were largely absent from the germinal layers and the intermediate zone. From 14 to 15 gw, however, MHC class II positive cells were also detected within germinal layers and in the corpus callosum, and these cells, which coexpressed CD68 antigen (a marker associated with phagocytosis), further populated the lower half of the telencephalon from 18 to 24 gw. These findings are discussed in relation to developmental events that take place during the second trimester within the wall of the telencephalon.

Isolation of enteric glia and establishment of transformed enteroglial cell lines from the myenteric plexus of adult rat

Although enteroglial cells (EGCs) may play a key role in the inflammatory response of the enteric nervous system, little is known about their immunophysiological properties. To facilitate further characterization of enteric glia, we have developed a novel method to isolate and purify EGCs from the myenteric plexus. Myenteric plexus preparations were enzymatically dissociated and EGCs purified by complement-mediated cytolysis of contaminating cells and transformed by retroviral gene transfer. Primary and transformed cells were characterized immunohistochemically and by dot-blot analysis. Functionally, c-fos mRNA expression was assessed in primary and transformed enteroglial cells. All cells displayed robust glial fibrillary acidic protein, S-100 and vimentin immunoreactivities, but no Thy-1.1, desmin, smooth muscle alpha-actin or C3 complement receptor immunoreactivity. This confirmed their enteroglial lineage and excluded contamination with other cell types. Both primary and transformed EGCs displayed little constitutive c-fos mRNA expression. This, however, could be upregulated by various stimuli, including proinflammatory cytokines. In summary, we present a novel method to purify EGCs from rat myenteric plexus for tissue culture and to establish transformed EGC lines that retain their glial nature and functional properties. Such cell lines are now available for physiological studies of the functional properties of enteric glia in vitro.

Behavioral recovery in 6-hydroxydopamine-lesioned rats by cotransduction of striatum with tyrosine hydroxylase and aromatic L-amino acid decarboxylase genes using two separate adeno-associated virus vectors

Parkinson's disease (PD) is characterized by the progressive loss of the dopaminergic neurons in the substantia nigra and a severe decrease in dopamine in the striatum. A promising approach to the gene therapy of PD is intrastriatal expression of enzymes in the biosynthetic pathway for dopamine. Tyrosine hydroxylase (TH) catalyzes the synthesis of L-dopa, which must be converted to dopamine by aromatic L-amino acid decarboxylase (AADC). Since the endogenous AADC activity in the striatum is considered to be low, coexpression of both TH and AADC in the same striatal cells would increase the dopamine production and thereby augment the therapeutic effects. In the present study, the TH gene and also the AADC gene were simultaneously transduced into rat striatal cells, using two separate adeno-associated virus (AAV) vectors, AAV-TH and AAV-AADC. Immunostaining showed that TH and AADC were coexpressed efficiently in the same striatal cells in vitro and in vivo. Moreover, cotransduction with these two AAV vectors resulted in more effective dopamine production and more remarkable behavioral recovery in 6-hydroxydopamine (6-OHDA)-lesioned rats, compared with rats receiving AAV-TH alone (p < 0.01). These findings suggest an alternative strategy for gene therapy of PD and indicate that the simultaneous transduction with two AAV vectors can extend their utility for potential gene therapy applications.

Chemical lesion of the inferior olive reduces [125I]sarcosine1-angiotensin II binding to AT2 receptors in the cerebellar cortex of young rats

In young rats, AT2 receptors and AT2 receptor mRNA are discretely localized in neurons of the inferior olive, with highest expression in the medial nucleus. We previously detected AT2 receptor binding, but not AT2 receptor mRNA, in the molecular layer of the cerebellar cortex. To determine whether AT2 receptors are expressed in climbing fiber terminals which arise to the molecular layer from the inferior olive and innervate Purkinje cells, we chemically destroyed olivary neurons of 2-week-old rats by intraperitoneal (i.p.) injection of the neurotoxin 3-acetylpyridine. Lesions of the inferior olive reduced [125I]Sar1-Ang II binding to AT2 receptors and AT2 receptor mRNA levels in this area by 50%, and produced a similar decrease in AT2 receptor binding in the molecular layer of the cerebellar cortex. The extent of binding reduction was similar 3 days and 7 days after the lesion. 3-Acetylpyridine lesions did not change [125I]Sar1-Ang II binding to AT1 receptors in the molecular layer of the cerebellar cortex or AT1 receptor mRNA levels in Purkinje cells. AT2 receptor binding and AT2 receptor mRNA levels in the deep cerebellar nuclei were also not affected by 3-acetylpyridine. Our results support the hypothesis that AT2 receptors are produced by inferior olivary neurons and transported through climbing fibers to the molecular layer of the cerebellar cortex. The high expression of AT2 receptors in the inferior olivary-cerebellar pathway during a crucial time in postnatal development of climbing fiber-Purkinje cell connectivity suggest a role of AT2 receptors in the development of this pathway.

Evidence for an active type of cell death with ultrastructural features distinct from apoptosis: the effects of 3-acetylpyridine neurotoxicity

3-Acetylpyridine is a niacinamide antagonist with potent neurotoxic properties in vitro and in vivo. 3-Acetylpyridine neurotoxicity was associated with positive DNA end-labelling and displayed features of active cell death without the ultrastructural changes of apoptotic cell death. After systemic administration in rats (70 mg/kg), we detected labelled nuclei in the inferior olive using in situ DNA end-labelling. However, the conventional chromatin stain did not show chromatin condensation or fragmentation and electron microscopy studies failed to reveal features of apoptosis. Although areas of condensed chromatin were present in some nuclei, cytoplasmic damage with extensive organelle swelling was the most prominent finding. In vitro, 3-acetylpyridine (0.1-1 mM) induced degeneration of cerebellar granule neurons in a concentration- and time-dependent manner. The protein synthesis inhibitor cycloheximide (10 micrograms/ml) and the transcriptional inhibitor actinomycin D (10 microM) protected against 3-acetylpyridine toxicity. In contrast, neither the free radical scavenger alpha-phenyl-N-tertbutylnitron (100 microM), nor glutathione ethyl ester (10-100 microM), N-acetyl-cysteine (10-200 microM) or 3-aminobenzamide (0.1-4 mM), an inhibitor of poly(ADP-ribose) synthesis, were effective. 3-Acetylpyridine-induced neuronal death in vitro was associated with positive in situ DNA labelling. However, DNA fragmentation could not be demonstrated prior to neuronal cell loss and no DNA "laddering" was detected by DNA gel electrophoresis. Correspondingly, no apoptotic nuclei were revealed upon electron microscopy but organelle swelling and extensive vacuolization, changes similar to autophagocytosis. In conclusion, 3-acetylpyridine induces an active form of cell death that required de novo protein synthesis but is distinct from apoptosis. A loss of glutathione accompanies, but does not precede, cell death.

ED2-positive perivascular cells act as neuronophages during delayed neuronal loss in the facial nucleus of the rat

Injection of Fluoro-Gold (FG) into the whisker pad of rats yields a stable retrograde labeling of facial motoneurons. After removal of 10 mm from the facial nerve the microglia phagocytose the FG-prelabeled dead neurons and assume the label. A subsequent brightfield immunostaining of the sections with HRP-DAB as end-product fully quenches the fluorescence of FG from all specifically stained structures (immunoquenching). Combining FG-labeling of neuronophages with immunoquenching, we recently described a population of enigmatic fluorescent cells, found in immediate vicinity to the motoneurons after the general neuronofugal migration of microglia. As the fluorescence of these cells was not quenched after a triple immunostaining with anti neuron-specific enolase, anti-GFAP, and OX-42 (quenching all fluorescence from neurons and glia), they seemed to represent a new, immunologically not identified neuronophage. Now we have further characterized this cell type. Following triple immunostaining, we tested a broad panel of mabs (OX-33, OX-19, OX-18, OX-6, R73, ED1, and ED2) to stain, quench fluorescence, and thus immunotype the unknown phagocytes. Only the mab ED2, the classical marker for perivascular cells, specifically stained the small round neuronophages. This surprising migration of perivascular cells toward decaying neurons was additionally tested and confirmed by intracerebroventricular application of FG prior to resection of the facial nerve Providing evidence for neuronophagia by ED2-positive cells, our results strongly support the hypothesis that the latter are the APC (antigen presenting cells) of the CNS.

Hypoxic-ischemic brain injury induces an acute microglial reaction in perinatal rats

Activated microglia may contribute to the progression of neuronal injury after a wide range of CNS insults. In this study, we used two complementary methods to evaluate acute changes in the morphology and regional distribution of microglia induced by a focal hypoxic-ischemic insult in 7-d-old (P7) rats. To elicit injury, P7 rats underwent right carotid ligation followed by 3 h of 8% O2 exposure; rats were killed 10 min to 5 d later (n > or = 3/group). A histochemical assay using Griffonia simplicifolia B4-isolectin enabled detection of both resting and activated microglia in tissue sections; vascular cells were also reactive. Activated microglia were also identified immunocytochemically using a macrophage-specific MAb, ED-1. In normal P7-12 brain, lectin, and ED-1 immunoreactive-activated microglia were concentrated in white matter; lectin-positive resting, ramified microglia were also detected throughout the gray and white matter. Subtle morphologic evidence of microglial activation was noted 10 min posthypoxia-ischemia in the lesioned right cerebral hemisphere; activated microglia began to accumulate within the next 4 h. Accumulation of lectin-positive activated microglia peaked at 2-4 d posthypoxia-ischemia. ED-1 immunoreactive-microglia were first noted 4 h after hypoxic-ischemic injury in the lesioned right hemisphere, and there was a corresponding increase in accumulation over the first 48 h posthypoxia-ischemia. In the left hemisphere, contralateral to the ligation, no increase in activated microglia were detected with either method. In brain sections where no neuronal injury was evident, activated microglia did not accumulate. These data demonstrate that perinatal hypoxic-ischemic brain injury induced rapid accumulation of activated microglia in hypoxic-ischemic forebrain.

Phagocytic microglia during delayed neuronal loss in the facial nucleus of the rat: time course of the neuronofugal migration of brain macrophages

The injection of Fluoro-Gold (FG) into the whisker pad of rats yields a stable fluorescent labeling of the motoneurons in the lateral facial subnucleus. Following resection of 8-10 mm of the facial nerve, the microglia phagocytose the FG-preloaded neurons and assume the label. Employing this vital labeling of microglia in situ we studied the fate of same after completion of phagocytic activity. Starting at 56 days post resection (DPR) the FG-labeled microglia spread out from the lateral facial subdivision and invaded the entire facial nucleus. The quantitative analysis of this redistribution of the fluorescent marker revealed a prolonged increase in the number of labeled microglia strictly proportional to the delayed loss of neurons. The differentiation between microglia and shrunken neurons was performed with the new method of immunoquenching: the staining of vibratome sections with anti-rat neuron-specific enolase (NSE) combined with an ABC-HRP kit and DAB as detector totally extinguished (quenched) all fluorescence from the pre-labeled facial motoneurons. The fluorescent microglia were additionally stained with GSA I-B4 and OX-42, which should completely quench all fluorescence in the section. However, a few small round cells, always closely opposed to neuronal perikarya, still fluoresced. These NSE-negative, GSA I-B4 and OX-42 negative, but fluorescent cells may represent a new, immunologically uncharacterized microglial cell type, that participates in neuronophagia.

Transitory disappearance of microglia during the regeneration of the lizard medial cortex

In normal lizards, microglial cells populate the medial cortex (a zone homologous to the hippocampal fascia dentata), with a preferential distribution along the border between the granular cell layer and the plexiform layers. Intraperitoneal injection of the neurotoxin 3-acetylpyridine (3AP) induces a selective lesion in the medial cortex with a rapid degeneration of the granular layer and its zinc-enriched axonal projection. Within 6-8 weeks, the granular layer is, however, repopulated by a new set of neurons generated in the subjacent ependyma and the cell debris is removed. The aim of this study was to determine to what extent microglia were involved in the scavenging processes during the regeneration process. To this end we studied the brains of regenerating lizards at different times after 3AP lesion, visualising microglial cells by the nucleoside diphosphatase (NDPase) histochemical reaction. Surprisingly, we found that stained microglial cells disappeared 6-8 hours after 3AP injection and remained absent until 10-15 days after injection. One month postlesion an increased population of microglial cells was found scattered throughout all plexiform layers of the cortex. Thorough examination of semithin and ultrathin sections confirmed the absence of microglia in the medial cortex of recent lesioned animals but the presence of an exuberant population after 1 month postlesion. In the tissue, phagocytotic scavenging was carried out by radial ependymocytes, not by microglia.

Early response of brain resident microglia to kainic acid-induced hippocampal lesions

We investigated the early response of microglia with complement and other proteins in well controlled rat central nervous system lesions. A selective neuronal degeneration in the hippocampal CA3 region was induced without direct tissue damage by an intraventricular injection of a small amount of kainic acid. As early as 1 h post injection, complement proteins C1q, C4, and C3 and immunoglobulin(Ig)G were found in the lesioned area. After 2 h, non-specific leakage of other plasma proteins occurred. By 3 h, reactive microglia gathered around the injured pyramidal neurons. Areas surrounding the lesions were depleted, on the other hand, indicating that these reactive microglia had originally resided in and migrated from such vacant areas. Upregulation of ICAM-1 expression by vascular endothelial cells commenced after 6 h. LFA-1-positive leucocytes were, then, accumulated in the vasculature, which was followed by an infiltration of leucocytes into the lesioned brain parenchyma. These results indicate that, following an acute neuronal injury, the response of the humoral factors such as complement proteins and IgG precedes the microglial reaction. Activation of vascular endothelial cells and subsequent infiltration of blood leucocytes occurs much later than the activation and migration of brain resident microglia. The origin of complement proteins and IgG in the lesioned brain parenchyma remains to be determined, although the production of complement proteins by microglia is suggested.