Response of striatal astrocytes to neuronal deafferentation: an immunocytochemical and ultrastructural study

AXL kinase-mediated astrocytic phagocytosis modulates outcomes of traumatic brain injury

Background

Complex changes in the brain microenvironment following traumatic brain injury (TBI) can cause neurological impairments for which there are few efficacious therapeutic interventions. The reactivity of astrocytes is one of the keys to microenvironmental changes, such as neuroinflammation, but its role and the molecular mechanisms that underpin it remain unclear.

Methods

Male C57BL/6J mice were subjected to the controlled cortical impact (CCI) to develop a TBI model. The specific ligand of AXL receptor tyrosine kinase (AXL), recombinant mouse growth arrest-specific 6 (rmGas6) was intracerebroventricularly administered, and selective AXL antagonist R428 was intraperitoneally applied at 30 min post-modeling separately. Post-TBI assessments included neurobehavioral assessments, transmission electron microscopy, immunohistochemistry, and western blotting. Real-time polymerase chain reaction (RT-PCR), siRNA transfection, and flow cytometry were performed for mechanism assessments in primary cultured astrocytes.

Results

AXL is upregulated mainly in astrocytes after TBI and promotes astrocytes switching to a phenotype that exhibits the capability of ingesting degenerated neurons or debris. As a result, this astrocytic transformation promotes the limitation of neuroinflammation and recovery of neurological dysfunction. Pharmacological inhibition of AXL in astrocytes significantly decreased astrocytic phagocytosis both in vivo and in primary astrocyte cultures, in contrast to the effect of treatment with the rmGas6. AXL activates the signal transducer and activator of the transcription 1 (STAT1) pathway thereby further upregulating ATP-binding cassette transporter 1 (ABCA1). Moreover, the supernatant from GAS6-depleted BV2 cells induced limited enhancement of astrocytic phagocytosis in vitro.

Conclusion

Our work establishes the role of AXL in the transformation of astrocytes to a phagocytic phenotype via the AXL/STAT1/ABCA1 pathway which contributes to the separation of healthy brain tissue from injury-induced cell debris, further ameliorating neuroinflammation and neurological impairments after TBI. Collectively, our findings provide a potential therapeutic target for TBI.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02201-3.

Role of astroglial toll-like receptors (TLRs) in central nervous system infections, injury and neurodegenerative diseases

Central nervous system (CNS) innate immunity plays essential roles in infections, neurodegenerative diseases, and brain or spinal cord injuries. Astrocytes and microglia are the principal cells that mediate innate immunity in the CNS. Pattern recognition receptors (PRRs), expressed by astrocytes and microglia, sense pathogen-derived or endogenous ligands released by damaged cells and initiate the innate immune response. Toll-like receptors (TLRs) are a well-characterized family of PRRs. The contribution of microglial TLR signaling to CNS pathology has been extensively investigated. Even though astrocytes assume a wide variety of key functions, information about the role of astroglial TLRs in CNS disease and injuries is limited. Because astrocytes display heterogeneity and exhibit phenotypic plasticity depending on the effectors present in the local milieu, they can exert both detrimental and beneficial effects. TLRs are modulators of these paradoxical astroglial properties. The goal of the current review is to highlight the essential roles played by astroglial TLRs in CNS infections, injuries and diseases. We discuss the contribution of astroglial TLRs to host defense as well as the dissemination of viral and bacterial infections in the CNS. We examine the link between astroglial TLRs and the pathogenesis of neurodegenerative diseases and present evidence showing the pivotal influence of astroglial TLR signaling on sterile inflammation in CNS injury. Finally, we define the research questions and areas that warrant further investigations in the context of astrocytes, TLRs, and CNS dysfunction.

PET Imaging of Crossed Cerebellar Diaschisis after Long-Term Cerebral Ischemia in Rats

Crossed cerebellar diaschisis (CCD) is a decrease of regional blood flow and metabolism in the cerebellar hemisphere contralateral to the injured brain hemisphere as a common consequence of stroke. Despite CCD has been detected in patients with stroke using neuroimaging modalities, the evaluation of this phenomenon in rodent models of cerebral ischemia has been scarcely evaluated so far. Here, we report the in vivo evaluation of CCD after long-term cerebral ischemia in rats using positron emission tomography (PET) imaging with 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). Imaging studies were combined with neurological evaluation to assess functional recovery. In the ischemic territory, imaging studies showed a significant decrease in glucose metabolism followed by a progressive recovery later on. Conversely, the cerebellum showed a contralateral hypometabolism from days 7 to 14 after reperfusion. Neurological behavior showed major impaired outcome at day 1 after ischemia followed by a significant recovery of the sensorimotor function from days 7 to 28 after experimental stroke. Taken together, these results suggest that the degree of CCD after cerebral ischemia might be predictive of neurological recovery.

Striatal Reinnervation Process after Acute Methamphetamine-Induced Dopaminergic Degeneration in Mice

Methamphetamine (METH), an amphetamine derivate, may increase the risk of developing Parkinson's disease (PD). Human and animal studies have shown that METH produces persistent dopaminergic neurotoxicity in the nigrostriatal pathway, despite initial partial recovery. To determine the processes leading to early compensation, we studied the detailed morphology and distribution of tyrosine hydroxylase immunoreactive fibers (TH-ir) classified by their thickness (types I-IV) before and after METH. Applying three established neurotoxic regimens of METH: single high dose (1 × 30 mg/kg), multiple lower doses (3 × 5 mg/kg) or (3 × 10 mg/kg), we show that METH primarily damages type I fibers (the thinner ones), and to a much lesser extend types II-IV fibers including sterile axons. The striatal TH terminal partial recovery process, consisting of a progressive regrowth increases in types II, III, and IV fibers, demonstrated by co-localization of GAP-43, a sprouting marker, was observed 3 days post-METH treatment. In addition, we demonstrate the presence of growth-cone-like TH-ir structures, indicative of new terminal generation as well as improvement in motor functions after 3 days. A temporal relationship was observed between decreases in TH-expression and increases in silver staining, a marker of degeneration. Striatal regeneration was associated with an increase in astroglia and decrease in microglia expression, suggesting a possible role for the neuroimmune system in regenerative processes. Identification of regenerative compensatory mechanisms in response to neurotoxic agents could point to novel mechanisms in countering the neurotoxicity and/or enhancing the regenerative processes.

Reactive astrocytes function as phagocytes after brain ischemia via ABCA1-mediated pathway

Astrocytes become reactive following various brain insults; however, the functions of reactive astrocytes are poorly understood. Here, we show that reactive astrocytes function as phagocytes after transient ischemic injury and appear in a limited spatiotemporal pattern. Following transient brain ischemia, phagocytic astrocytes are observed within the ischemic penumbra region during the later stage of ischemia. However, phagocytic microglia are mainly observed within the ischemic core region during the earlier stage of ischemia. Phagocytic astrocytes upregulate ABCA1 and its pathway molecules, MEGF10 and GULP1, which are required for phagocytosis, and upregulation of ABCA1 alone is sufficient for enhancement of phagocytosis in vitro. Disrupting ABCA1 in reactive astrocytes result in fewer phagocytic inclusions after ischemia. Together, these findings suggest that astrocytes are transformed into a phagocytic phenotype as a result of increase in ABCA1 and its pathway molecules and contribute to remodeling of damaged tissues and penumbra networks.

Astrocytic phagocytosis has been shown to play a role in synaptic pruning during development, but whether adult astrocytes possess phagocytic ability is unclear. Here the authors show that following brain ischemia, reactive astrocytes become phagocytic and engulf debris via the ABCA1 pathway.

Chronic Exposure to Intra-Amniotic Lipopolysaccharide Affects the Ovine Fetal Brain

OBJECTIVE

Fetal brain injury is associated with chorioamnionitis, which is often present without signs of overt infection or fetal compromise. We aimed to determine if prolonged exposure to intrauterine inflammation caused by intra-amniotic infusion of lipopolysaccharide (LPS) would affect the fetal brain.

METHODS

At 80 days of pregnancy ewes bearing singletons had osmotic pumps implanted intra-amniotically to infuse Escherichia coli LPS (055:B5; n = 8) or saline (n = 7) for 28 days. At delivery (110 days), umbilical arterial blood and chorioamnion were assessed for inflammation; cytokine concentrations (interleukin [IL]-6 and IL-8) in amniotic fluid and fetal and maternal plasma were measured. The fetal cerebral hemispheres were examined for gross anatomical changes and the number of activated microglia/macrophages, astrocytes, and oligodendrocytes estimated after immunohistochemical staining.

RESULTS

Intra-amniotic administration of LPS caused chorioamnionitis, fetal leucocytosis, and a moderate to extensive infiltration of activated microglia/macrophages in the subcortical white matter in six of eight fetuses; the remaining two fetuses were less affected. Within these focal regions of damage there was an attenuation of astrocytic processes, axonal injury, and a reduction in the number of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) immunoreactive oligodendrocytes in areas of extensive focal damage. In control fetuses there was mild (3/7) or no infiltration of activated microglia/macrophages in the subcortical white matter. Overall the infiltration of activated microglia/macrophages in the white matter was significantly greater in LPS-exposed fetuses compared to controls. In regions devoid of injury, the number of oligodendrocytes and astrocytes was not different between groups, nor was there a difference in the volume of cerebral white matter or density of blood vessels within the white matter. Amniotic fluid IL-6 and IL-8, and maternal plasma IL-8 concentrations were significantly increased by LPS infusion.

CONCLUSIONS

An increase in inflammatory cells and axonal disruption in the subcortical white matter of the fetal brain can accompany chorioamnionitis induced by intra-amniotic administration of LPS, but cystic lesions do not occur. Thus, the effect on the fetal brain is milder than that reported from animal models of acute fetal/intrauterine infection.

Age determination of brain contusions

In 104 individuals who had sustained traumatic brain injury, the course of traumatically induced morphological changes was investigated immunohistochemically during the first 30 weeks after the trauma. Regarding the inflammatory cell reaction in human cortical contusions, CD15-labeled granulocytes were detectable within 10 minutes following brain injury, whereas significantly increased numbers of nuclear leukocytes occurred after a postinfliction interval of at least 1.1 days (leukocyte common antigen), 2 days (CD3), or 3.7 days (UCHL-1), respectively. A positive nuclear staining for the proliferation marker MIB-1 by cerebral macrophages could be observed as early as 3 days after the injury and regularly in cases with a survival between 7 and 11 days. Injury-induced glial staining reactions could be demonstrated, at the earliest, after a postinfliction interval of 3 hours for α1-antichymotrypsin, 22 hours for vimentin, 1 day for glial fibrillary acidic protein, and 7 days for tenascin. Regarding the vascular response to brain injury, a significantly increased immunoreactivity could be detected in cortical contusions with a wound age of at least 3 hours for factor VIII, 1.6 days for tenascin, and 6.8 days for thrombomodulin, whereas the immunostaining for laminin and type IV collagen was regularly whereas the immunostaining for laminin and type IV collagen was regularly positive even in the vascular endothelium of ininjured brain tissue.

Amphotericin B induces glial cell line-derived neurotrophic factor in the rat brain

Amphotericin B (AmB) is a polyene antifungal drug and is reported to be one of a few reagents having therapeutic effects on prion diseases, that is, a delay in the appearance of clinical signs and prolongation of the survival time in an animal model. In prion diseases, glial cells have been suggested to play important roles; however, the therapeutic mechanism of AmB on prion diseases remains elusive. We have previously reported that AmB changed the expression of neurotrophic factors in microglia and astrocytes (Motoyoshi et al., 2008, Neurochem. Int. 52, 1290–1296; Motoyoshi-Yamashiro et al., 2013, ibid. 63, 93–100). These results suggested that neurotrophic factors derived from glial cells might be involved in the therapeutic mechanism of AmB. In the present study, we examined immunohistochemically the effects of AmB on the expression of neurotrophic factors in the rat brain. We found that direct injection of AmB into the striatum significantly enhanced the expression of glial cell line-derived neurotrophic factor protein. Amphotericin B also increased the expressions of CD11b and glial fibrillary acidic protein, markers of microglia and astrocytes, respectively. Moreover, expressions of the two neurotrophic factors by AmB were co-localized with the expression of CD11b or glial fibrillary acidic protein. These results suggest that AmB in vivo might also activate glial cells and induce the production of neurotrophic factors protecting neurons in prion diseases.

Mechanical stress-induced reactive gliosis in the auditory nerve and cochlear nucleus

OBJECT

Hearing levels following microsurgical treatment gradually deteriorate in a number of patients treated for vestibular schwannoma (VS), especially in the subacute postoperative stage. The cause of this late-onset deterioration of hearing is not completely understood. The aim of this study was to investigate the possibility that reactive gliosis is a contributory factor.

METHODS

Mechanical damage to nerve tissue is a feature of complex surgical procedures. To explore this aspect of VS treatment, the authors compressed rat auditory nerves with 2 different degrees of injury while monitoring the compound action potentials of the auditory nerve and the auditory brainstem responses. In this experimental model, the axons of the auditory nerve were quantitatively and highly selectively damaged in the cerebellopontine angle without permanent compromise of the blood supply to the cochlea. The temporal bones were processed for immunohistochemical analysis at 1 week and at 8 weeks after compression.

RESULTS

Reactive gliosis was induced not only in the auditory nerve but also in the cochlear nucleus following mechanical trauma in which the general shape of the auditory brainstem response was maintained. There was a substantial outgrowth of astrocytic processes from the transitional zone into the peripheral portion of the auditory nerve, leading to an invasion of dense gliotic tissue in the auditory nerve. The elongated astrocytic processes ran in parallel with the residual auditory neurons and entered much further into the cochlea. Confocal images disclosed fragments of neurons scattered in the gliotic tissue. In the cochlear nucleus, hypertrophic astrocytic processes were abundant around the soma of the neurons. The transverse diameter of the auditory nerve at and proximal to the compression site was considerably reduced, indicating atrophy, especially in rats in which the auditory nerve was profoundly compressed.

CONCLUSIONS

The authors found for the first time that mechanical stress to the auditory nerve causes substantial reactive gliosis in both the peripheral and central auditory pathways within 1-8 weeks. Progressive reactive gliosis following surgical stress may cause dysfunction in the auditory pathways and may be a primary cause of progressive hearing loss following microsurgical treatment for VS.

Electrospun nanofibrous materials for tissue engineering and drug delivery

The electrospinning technique, which was invented about 100 years ago, has attracted more attention in recent years due to its possible biomedical applications. Electrospun fibers with high surface area to volume ratio and structures mimicking extracellular matrix (ECM) have shown great potential in tissue engineering and drug delivery. In order to develop electrospun fibers for these applications, different biocompatible materials have been used to fabricate fibers with different structures and morphologies, such as single fibers with different composition and structures (blending and core-shell composite fibers) and fiber assemblies (fiber bundles, membranes and scaffolds). This review summarizes the electrospinning techniques which control the composition and structures of the nanofibrous materials. It also outlines possible applications of these fibrous materials in skin, blood vessels, nervous system and bone tissue engineering, as well as in drug delivery.

Clusterin increases post-ischemic damages in organotypic hippocampal slice cultures

Clusterin or apolipoprotein J is a heterodimeric glycoprotein which is known to be increased during tissue involution in response to hormonal changes or injury and under circumstances leading to apoptosis. Previous studies in wild-type (WT) and clusterin-null (Clu-/-) mice indicated a protective role of clusterin over-expression in astrocytes lasting up to 90 days post-ischemia. However, in in vitro and in vivo models of neonatal hypoxia-ischemia, clusterin exacerbates necrotic cell death. We developed recombinant forms of clusterin and examined their effect on propidium iodide uptake, neuronal and synaptic markers as well as electrophysiological recordings in hippocampal slice cultures from Clu-/- and WT mice subjected to oxygen-glucose deprivation (OGD). WT mice displayed a marked up-regulation of clusterin associated with electrophysiological deficits and dramatic increase of propidium iodide uptake 5 days post-OGD. Immunocytochemical and western blot analyses revealed a substantial decrease of neuronal nuclei and synaptophysin immunoreactivity that predominated in WT mice. These findings contrasted with the relative post-OGD resistance of Clu-/- mice. The addition of biologically active recombinant forms of human clusterin for 24 h post-OGD led to the abolishment of the ischemic tolerance in Clu-/- slices. This deleterious effect of clusterin was reverted by the concomitant administration of the NMDA receptor antagonist, d-2-amino-5-phosphonopentanoate. The present data indicate that in an in vitro model of ischemia characterized by the predominance of NMDA-mediated cell death, clusterin exerts a negative effect on the structural integrity and functionality of hippocampal neurons.

Clusterin expression during fetal and postnatal CNS development in mouse

Clusterin (or apolipoprotein J) is a widely distributed multifunctional glycoprotein involved in CNS plasticity and post-traumatic remodeling. Using biochemical and morphological approaches, we investigated the clusterin ontogeny in the CNS of wild-type (WT) mice and explored developmental consequences of clusterin gene knock-out in clusterin null (Clu-/-) mice. A punctiform expression of clusterin mRNA was detected through the hypothalamic region, neocortex and hippocampus at embryonic stages E14/E15. From embryonic stage E16 to the first week of the postnatal life, the vast majority of CNS neurons expressed low levels of clusterin mRNA. In contrast, a very strong hybridizing signal mainly localized in pontobulbar and spinal cord motor nuclei was observed from the end of the first postnatal week to adulthood. Astrocytes expressing clusterin mRNA were often detected through the hippocampus and neocortex in neonatal mice. Real-time polymerase chain amplification and clusterin-immunoreactivity dot-blot analyses indicated that clusterin levels paralleled mRNA expression. Comparative analyses between WT and Clu-/- mice during postnatal development showed no significant differences in brain weight, neuronal, synaptic and astrocyte markers as well myelin basic protein expression. However, quantitative estimation of large motor neuron populations in the facial nucleus revealed a significant deficit in motor cells (-16%) in Clu-/- compared with WT mice. Our data suggest that clusterin expression is already present in fetal life mainly in subcortical structures. Although the lack of this protein does not significantly alter basic aspects of the CNS development, it may have a negative impact on neuronal development in certain motor nuclei.

Plasticity of basal ganglia neurocircuitries following perinatal asphyxia: effect of nicotinamide

The potential neuroprotection of nicotinamide on the consequences of perinatal asphyxia was investigated with triple organotypic cultures. Perinatal asphyxia was induced in vivo by immersing foetuses-containing uterine horns removed from ready-to-deliver rats into a water bath for 20 min. Sibling caesarean-delivered pups were used as controls. Three days later tissue from substantia nigra, neostriatum and neocortex was dissected and placed on a coverslip. After a month, the cultures were processed for immunocytochemistry and phenotyped with markers against the NMDA receptor subunit NR1, tyrosine hydroxylase (TH), or neuronal nitric oxide synthase (nNOS). Some cultures were analysed for cell viability. Nicotinamide (0.8 mmol/kg, i.p.) or saline was administered to asphyxia-exposed and caesarean-delivered control pups 24, 48 and 72 h after birth. Perinatal asphyxia produced a decrease of cell viability in substantia nigra, but not in neostriatum or neocortex. Immunocytochemistry confirmed the vulnerability of the substantia nigra, demonstrating that there was a significant decrease in the number of NR1 and TH-positive (+) cells/mm2, as well as a decrease in the length of TH+ processes, suggesting neurite atrophy. In control cultures, many nNOS+ cells were seen, with different features, regional distribution and cell body sizes. Following perinatal asphyxia, there was an increase in the number of nNOS+ cells/mm2 in substantia nigra, versus a decrease in neostriatum including reduced neurite length, and no apparent changes in neocortex. The main effect of nicotinamide was seen in the neostriatum, preventing the asphyxia-induced decrease in the number of nNOS+ cells and neurite length. Nicotinamide also prevented the effect of perinatal asphyxia on TH-positive neurite length. The present results support the idea that nicotinamide can prevent the effects produced by a sustained energy-failure condition, as occurring during perinatal asphyxia.

Hypoxic-ischemic injury in the immature brain--key vascular and cellular players

Over the past decade, much has been learned about the cellular and molecular mechanisms underlying hypoxic-ischemic (H-I) injury in the preterm human brain. The pathogenesis of H-I brain injury is now understood to be multifactorial and quite complex, depending on (i) the severity, intensity and timing of asphyxia, (ii) selective ischemic vulnerability, (iii) the degree of maturity of the brain, and (iv) the characteristics of the ensuing reoxygenation/reperfusion phase. Each of these factors has differential effects on the distinct cell populations in the brain, with certain specific cell types being particularly vulnerable in the developing brain. In this review, we discuss the role of the blood vessels and the distinct cell populations, which are the mayor constitutive elements of the immature brain, in the pathophysiology of H-I lesion. The presence of fragile and poorly anastomosed blood vessels and the existence of disturbances in the blood-brain barrier alter blood flow, vascular tone and nutrient delivery. Brain cells are sensitive to the overstimulation of neurotransmitter receptors, particularly glutamate receptors, which can provoke excitotoxicity leading to the death of neurons and other cells such as astrocytes and oligodendrocyte progenitors. Microglial activation by means of excitatory amino acids and by leukocyte migration initiates the inflammatory response giving rise to an increase in regional cerebral blood flow and promoting astrocyte and oligodendrocyte injuries. A better understanding of these aspects of H-I injury will contribute to more efficient strategies for the management of the associated damage.

Progressive Reparative Gliosis in Aged Hosts and Interferences with Neural Grafts in an Animal Model of Huntington's Disease

1. Neural transplantation in Huntington's diseased patients is currently the only approach in the treatment of this neurodegenerative disorder. The clinical trial, unfortunately, includes only a small number of patients until now, since many important questions have not been answered yet. One of them is only mild to moderate improvement of the state in most of grafted patients. 2. We examined the morphological correlates in the response to intrastriatal grafting of fragments of foetal rat ventral mesencephalic tissue 1 month after transplantation in male Wistar rats within varying durations (from 2 to 38 weeks) of experimentally induced neurodegenerative process of the striatum (used as a model of Huntington's disease). Our goal was to determine the impact of advanced striatal damage and gliosis on the graft viability and host-graft integration. 3. The findings can be summarized as follows: The progressive reactive gliosis, which is not able to compensate continual reduction of the grey matter leading to an extensive atrophy of the striatum in a long-term lesions, results in formation of the compact glial network. This tissue cannot be considered the suitable terrain for successful graft development and formation of host-graft interconnections. 4. The progression of irreversible morphological changes in long-lasting neurodegenerative process within the striatum can be supposed one of the important factors, which may decrease our prospect of distinct improvement after neural grafting in patients in advanced stage of Huntington's disease, who still remain the leading group in clinical trials.

Sustained astrocytic clusterin expression improves remodeling after brain ischemia

Clusterin is a glycoprotein highly expressed in response to tissue injury. Using clusterin-deficient (Clu-/-) mice, we investigated the role of clusterin after permanent middle cerebral artery occlusion (MCAO). In wild-type (WT) mice, clusterin mRNA displayed a sustained increase in the peri-infarct area from 14 to 30 days post-MCAO. Clusterin transcript was still present up to 90 days post-ischemia in astrocytes surrounding the core infarct. Western blot analysis also revealed an increase of clusterin in the ischemic hemisphere of WT mice, which culminates up to 30 days post-MCAO. Concomitantly, a worse structural restoration and higher number of GFAP-reactive astrocytes in the vicinity of the infarct scar were observed in Clu-/- as compared to WT mice. These findings go beyond previous data supporting a neuroprotective role of clusterin in early ischemic events in that they demonstrate that this glycoprotein plays a central role in the remodeling of ischemic damage.

Cholesterol homeostasis and the pathophysiology of Alzheimer's disease

The past 4 years have seen a growing interest in cholesterol metabolism and its relationship to Alzheimer's disease. From the first report linking cholesterol and beta-amyloid metabolisms to the recent positive report on the use of atorvastatin (Lipitor, Pfizer Inc.), a cholesterol-lowering drug, in mild-to-moderate Alzheimer's disease, this review examines the scientific progress pertaining to etiopathology of Alzheimer's disease over the past 15 years and the central role of lipids in this field of research. The role of key proteins involved in this metabolic pathway such as apolipoprotein E, lipoprotein lipase, caveolin, hydroxy-methylglutaryl Coenzyme A reductase, low-density lipoprotein receptors, cholesterol 24-hydroxylase, acyl-coenzyme A:cholesterol acyltransferase and beta-amyloid are discussed.

Changes of parvalbumin immunoreactive neurons and GFAP immunoreactive astrocytes in the rat lateral geniculate nucleus following monocular enucleation

The expression of calcium binding proteins (CaBPs) has been linked to protection of neurons. The present study investigated the effects of monocular enucleation on the distribution of parvalbumin immunoreactive (PV-IR) neurons and glial fibrillary acidic protein immunoreactive (GFAP-IR) astrocytes in both the dorsal (dLGN) and ventral (vLGN) regions of the lateral geniculate nucleus (LGN). Our results demonstrated an increase in PV-IR neuronal density on the contralateral vLGN at 1-week post-enucleation (PE), which was maintained without significant change for 12 weeks. By contrast, PV-IR neurons in dLGN decreased significantly at all time point examined. The number of GFAP-IR astrocytes showed an initial increase from 1 to 4 weeks PE and then gradually decreased until 48 weeks in both regions of the LGN with contralateral side predominance. The present results suggest that monocular enucleation results in variable expression of PV-IR neurons and GFAP-IR astrocytes in the LGN complex, which may play an important role in neuronal degeneration and neuroplasticity of the rat visual system.

Inflammatory responses of the substantia nigra after acute hypoxia in neonatal rats

The neocortex and the striatum are the brain regions most known to be particularly vulnerable to acute insults like hypoxia or ischemia. In this work, we assess the possibility of cellular damage to the substantia nigra (SN) after hypoxia-reoxygenation in the new born rat. The aim of the present paper was to evaluate the expression of growth factor IGF-I, and growth factor binding proteins IGFBP-3 and IGFBP-5 genes and induction of NOS family members (nNOS, eNOS and iNOS) and TNF-alpha genes together with glia activation, in the SN at 5 and 48 h after severe hypoxia in the 7 day-old rat, a model for the term human fetus. At early time, while IGFs remain unchanged, we found a transient increase in eNOS and nNOS. Two days after the injury, nNOS expression remained high, iNOS and TNF-alpha increased and also GFAP protein expression was observed together with a profusion of reactive astrocytes distributed throughout the SN. This study on the acute effects of hypoxia on the developing brain provides additional insights into the vulnerability of the SN, a brain region involved in neurodegenerative pathologies.

Differential regulation of the growth-associated proteins GAP-43 and superior cervical ganglion 10 in response to lesions of the cortex and substantia nigra in the adult rat

Investigation of the elements underlying synapse replacement after brain injury is essential for predicting the neural compensation that can be achieved after various types of damage. The growth-associated proteins superior cervical ganglion-10 and growth-associated protein-43 have previously been linked with structural changes in the corticostriatal system in response to unilateral deafferentation. To examine the regulation of this response, unilateral cortical aspiration lesion was carried out in combination with ipsilateral 6-hydroxydopamine lesion of the substantia nigra, and the time course of the contralateral cortical molecular response was followed. Unilateral cortical aspiration lesion in rats corresponds with an upregulation of superior cervical ganglion-10 mRNA at 3 and 10 days post-lesion, and protein, sustained from three to at least 27 days following lesion. With the addition of substantia nigra lesion, the response shifts to an upregulation of growth-associated protein-43 mRNA at 3 and 10 days post-lesion, and protein after 10 days. Nigral lesion alone does not alter contralateral expression of either gene. Likewise, motor function assessment using the rotorod test revealed no significant long-term deficits in animals that sustained only nigrostriatal damage, but cortical lesion was associated with a temporary deficit which was sustained when nigrostriatal input was also removed. Growth-associated protein-43 and superior cervical ganglion-10, two presynaptic genes that are postulated to play roles in lesion-induced sprouting, are differentially upregulated in corticostriatal neurons after cortical versus combined cortical/nigral lesions. The shift in contralateral gene response from superior cervical ganglion-10 to growth-associated protein-43 upregulation and associated behavioral deficit following combined cortical and nigral denervation suggest that nigrostriatal afferents regulate cortical lesion-induced gene expression and ultimate functional outcome.

Debris of "dark" (compacted) neurones are removed from an otherwise undamaged environment mainly by astrocytes via blood vessels

By means of a condenser-discharge electric shock paradigm, "dark" granule neurones were momentarily produced in a sporadic distribution among normal ones in the otherwise undamaged (non-necrotic, non-excitotoxic, non-inflammatory or non-contused) hippocampal dentate gyri of the rat brain. In the electron microscope, the ultrastructural elements of the affected neurones remained undamaged but turned markedly electron-dense and the distances between them became strikingly reduced (compaction). A proportion of such neurones recovered in 1 day while others died. During the first week of survival, the dead "dark" granule neurones retained the compacted and electron-dense ultrastructure, but underwent cytoplasmic convolution and fragmentation. The fragments were enclosed by membranes and separated from each other and from the intact neuropil by astrocytic processes containing an excess of glycogen particles. Neither proliferation of microglial cells nor infiltration of haematogenous macrophages was observed. A few fragments were taken over by resting microglial cells, while the majority was engulfed by astrocytes. The latter transported the engulfed fragments, either unchanged or digested to various degrees, to capillaries, arterioles and venules. Thereafter, the astrocyte-engulfed neuronal fragments, as well as their partly or completely digested remnants, were either transferred to phagocytotic pericytes or discharged into vascular lumina.

Involvement of interferon-gamma and its receptor in the activation of astrocytes in the mouse hippocampus following entorhinal deafferentation

The activation of glial cells has been thought to be a universal and important reaction to trauma and pathology in the mammalian central nervous system. The mechanism of glial activation is not completely clear to date, but numerous cytokines have been demonstrated to effectively influence the process in vitro and in vivo. Here we reported the axotomy-induced upregulation of interferon-gamma (IFN-gamma) receptor mRNA in the mouse hippocampus following transections of the entorhinal afferents. Northern blot analysis showed that the transcripts of IFN-gamma receptor were upregulated in a transient manner in the deafferented mouse hippocampus. In situ hybridization confirmed the temporal upregulation of IFN-gamma receptor mRNA specifically in the denervated areas of the mouse hippocampus, which showed that the expression of IFN-gamma receptor mRNA rose slightly at 2 days postlesion, increased remarkably at 3 days postlesion, nearly reached the maximum at 7 days postlesion, and almost returned to control levels at 15 days postlesion. Double labeling further proved that the upregulated IFN-gamma receptor mRNA was confined to reactive astrocytes. At 2 and 3 days postlesion, we also observed the expression of IFN-gamma mRNA by a small number of cells in the denervated areas. We noted that the upregulation of both IFN-gamma and its receptor expression coincided spatiotemporally with astroglial activation, suggesting the potential involvement of IFN-gamma and its receptor in the activation process of astrocytes in the hippocampus following entorhinal deafferentation.

Glial responses associated with dopaminergic striatal reinnervation following lesions of the rat substantia nigra

Lesioning of dopaminergic substantia nigra pars compacta (SNpc) neurons leads to depletion of dopamine (DA) and dopaminergic axons in the dorsal striatum, followed by subsequent compensatory sprouting of dopaminergic fibers and striatal reinnervation. In this study, the response of striatal glia (microglia and astroglia) was compared with the degeneration and regeneration of dopaminergic axons following SNpc lesions. Following partial SNpc lesions, density of dopamine transporter (DAT) immunoreactive (-ir) terminals in the dorsal striatum returned to normal within 16 weeks of injury, suggesting that dopaminergic reinnervation of the striatum was complete. In conjunction, the glial responses in the dorsal striatum consisted of two peaks. The first peak in glial density occurred immediately after lesioning, peaking at 7 days, implying that it was likely to be associated with removal of debris from degenerating terminals. The second glial response commenced 8 weeks after lesioning and peaked some time after 16 weeks. The time of onset of the second peak suggests that it may be associated with the establishment of synapses rather than with axonal guidance.

Synapse replacement in the striatum of the adult rat following unilateral cortex ablation

Defining the selective pattern of synapse replacement that occurs in different areas of the damaged brain is essential for predicting the limits of functional compensation that can be achieved after various types of brain injury. Here we describe the time course of dendritic reorganization, spine loss and recovery, and synapse replacement in the striatum following a unilateral cortex ablation. We found that the time course for the transient loss and recovery of dendritic spines on medium spiny I (MSI) neurons, the primary postsynaptic target for corticostriatal axons, paralleled the time course for the removal of degenerating axon terminals from the neuropil and the formation of new synapses on MSI neurons. Reinnervation of the deafferented striatum occurred chiefly by axon terminals that formed asymmetric synapses with dendritic spines of MSI neurons, and the mean density of asymmetric synapses recovered to 86% of the sham-operated rat value by 30 days postlesion. In addition, the synaptic circuitry of the reconstructed striatum was characterized by an increase in the number of multiple synaptic boutons (MSBs), i.e., presynaptic axon terminals that make contact with more than one dendritic spine. Whether the postsynaptic contacts of MSBs are formed with the dendritic spines of the same or a different parent dendrite in the striatum is unknown. Nevertheless, these data suggest that the formation of MSBs is an essential part of the compensatory response to the loss of input from the ipsilateral cortex following the aspiration lesion and may serve to modulate activity-dependent adaptive changes in the reconstructed striatum that can lead to functional recovery.

Astrocytes accumulate A beta 42 and give rise to astrocytic amyloid plaques in Alzheimer disease brains

beta-Amyloid(1-42) (A beta 42), a major component of amyloid plaques, accumulates within pyramidal neurons in the brains of individuals with Alzheimer's disease (AD) and Down syndrome. In brain areas exhibiting AD pathology, A beta 42-immunopositive material is observed in astrocytes. In the present study, single- and double-label immunohistochemistry were used to reveal the origin and fate of this material in astrocytes. Our findings suggest that astrocytes throughout the entorhinal cortex of AD patients gradually accumulate A beta 42-positive material and that the amount of this material correlates positively with the extent of local AD pathology. A beta 42-positive material within astrocytes appears to be of neuronal origin, most likely accumulated via phagocytosis of local degenerated dendrites and synapses, especially in the cortical molecular layer. The co-localization of neuron-specific proteins, alpha 7 nicotinic acetylcholine receptor and choline acetyltransferase, in A beta 42-burdened, activated astrocytes supports this possibility. Our results also suggest that some astrocytes containing A beta 42-positive deposits undergo lysis, resulting in the formation of astrocyte-derived amyloid plaques in the cortical molecular layer in brain regions showing moderate to advanced AD pathology. These astrocytic plaques can be distinguished from those arising from neuronal lysis by virtue of their smaller size, their nearly exclusive localization in the subpial portion of the molecular layer of the cerebrocortex, and by their intense glial fibrillary acidic protein immunoreactivity. Overall, A beta 42 accumulation and the selective lysis of A beta 42-burdened neurons and astrocytes appear to make a major contribution to the observed amyloid plaques in AD brains.

Neuroprotection by memantine against neurodegeneration induced by beta-amyloid(1-40)

Progressive neuronal loss and cognitive decline in Alzheimer's disease (AD) might be aggravated by beta-amyloid-enhanced excitotoxicity. Memantine is an uncompetitive NMDA receptor antagonist under clinical development for the treatment of AD. Memantine has neuroprotective actions in several in vitro and in vivo models. In the present study, we determined whether memantine protected against beta-amyloid induced neurotoxicity and learning impairment in rats. Twenty Sprague-Dawley rats received vehicle or vehicle plus memantine (steady-state plasma concentrations of 2.34+/-0.23 microM, n=10) s.c. by osmotic pump for 9 days. After 2 days of treatment, 2 microl of water containing beta-amyloid 1-40 [Abeta(1-40)] were injected into the hippocampal fissure. On the ninth day of treatment, animals were sacrificed, and morphological and immunohistochemical techniques were used to determine the extent of neuronal degeneration and astrocytic and microglial activation in the hippocampus. Psychomotor activity and spatial discrimination were tested on the eighth day of treatment. Abeta(1-40), but not water, injections into hippocampus led to neuronal loss in the CA1 subfield, evidence of widespread apoptosis, and astrocytic and microglial activation and hypertrophy. Memantine treated animals had significant reductions in the amount of neuronal degeneration, pyknotic nuclei, and GFAP immunostaining as compared with vehicle treated animals. These data suggest that memantine, at therapeutically relevant concentrations, can protect against neuronal degeneration induced by beta-amyloid.

Dendritic spine pathology: cause or consequence of neurological disorders?

Altered dendritic spines are characteristic of traumatized or diseased brain. Two general categories of spine pathology can be distinguished: pathologies of distribution and pathologies of ultrastructure. Pathologies of spine distribution affect many spines along the dendrites of a neuron and include altered spine numbers, distorted spine shapes, and abnormal loci of spine origin on the neuron. Pathologies of spine ultrastructure involve distortion of subcellular organelles within dendritic spines. Spine distributions are altered on mature neurons following traumatic lesions, and in progressive neurodegeneration involving substantial neuronal loss such as in Alzheimer's disease and in Creutzfeldt-Jakob disease. Similarly, spine distributions are altered in the developing brain following malnutrition, alcohol or toxin exposure, infection, and in a large number of genetic disorders that result in mental retardation, such as Down's and fragile-X syndromes. An important question is whether altered dendritic spines are the intrinsic cause of the accompanying neurological disturbances. The data suggest that many categories of spine pathology may result not from intrinsic pathologies of the spiny neurons, but from a compensatory response of these neurons to the loss of excitatory input to dendritic spines. More detailed studies are needed to determine the cause of spine pathology in most disorders and relationship between spine pathology and cognitive deficits.

Effects of red nucleus ablation and exogenous neurotrophin-3 on corticospinal axon terminal distribution in the adult rat

Collateral sprouting of undamaged descending axons is one potential mechanism for recovery of function after incomplete spinal cord injury. In this study, we have investigated whether terminals of the intact corticospinal tract in the rat would sprout following ablation of a parallel descending pathway, the rubrospinal tract. No sprouting was detected after this injury alone. However, the combination of rubrospinal tract ablation with administration of 100ng neurotrophin-3 to neurons of the corticospinal tract resulted in marked increased density of corticospinal innervation in the superficial dorsal horn. There was no effect of administration of neurotrophin-3 alone and increase in axon density was not detected in the deep dorsal horn. These results imply that spontaneous sprouting of undamaged corticospinal axons does not occur following ablation of a parallel tract system, although collateral sprouting can be induced through a combination of the lesion plus exogenous growth factor. Induced change in corticospinal terminal density is detected in the superficial dorsal horn only, supporting the hypothesis that this is an area particularly supportive of circuit reorganisation.

Microglial activation, emergence of ED1-expressing cells and clusterin upregulation in the aging rat CNS, with special reference to the spinal cord

With advancing age, the incidence of neuronal atrophy and dystrophy increases and, in parallel, behavioural sensorimotor impairment becomes overt. Activated microglia has been implicated in cytotoxic and inflammatory processes in neurodegenerative diseases as well as during aging. Here we have used immunohistochemistry and in situ hybridization to examine the expression of OX42, ED1, ED2, GFAP and clusterin in CNS of young adult and behaviourally tested aged rats (30-month-old), to study the occurrence of activated microglia/ED1 positive macrophages in senescence and to what extent this correlates with astrogliosis and signs of sensorimotor impairment among the individuals. The results show a massive region-specific increase in activated microglia and ED1 expressing cell profiles in aged rats. The infiltration was most prominent in the spinal cord dorsal columns, including their sensory relay nuclei, and the outer portions of the lateral and ventral columns. At such sites the occurrence of macrophages coincided with increased levels of GFAP and positive correlations were evident between the labeling for, on the one hand, OX42 and, on the other, GFAP and ED1. Also, the ventral and dorsal roots were heavily infiltrated by ED1 positive cells. The signs of gliosis were most pronounced among aged rats with advanced sensorimotor impairment. In contrast, the grey matter of aged rats showed very few activated microglia/ED1 labeled cells despite signs of focal astrogliosis. ED2 expression was confined to perivascular cells and leptominges with a similar labeling pattern in young and aged rats. In aged rats increased expression of clusterin was observed in GFAP-immunoreactive profiles of the white matter only. It is suggested that this increase may reflect a response to degenerative/inflammatory processes.

Upregulation of glial clusterin in brains of patients with AIDs

Since clusterin (CLU) production in reactive astrocytes may be neuroprotective, we examined its distribution in AIDS brains where brain injury and reactive astrocytosis are common. The relative area and number of CLU-positive astrocytes, as well as their percent total of all white matter glia, significantly increased in AIDS brains with and without HIV encephalitis (P<0.05). Proliferation markers were absent. In contrast, the relative area and number of GFAP-positive astrocytes and their percent of all white matter glia, increased in some cases but the mean increases were not significant. Clusterin is sensitive marker of glial reactivity in AIDS brains and its enhanced expression was not dependent on increases in GFAP.

Neuronal death, not axonal degeneration, results in significant gliosis within the cochlear nucleus of adult chickens

Injury to the central nervous system initiates a series of events that leads to neuronal cell death and glial activation. Astrocytes respond to damage and disease by becoming hyperplastic and hypertrophied. This 'reactive gliosis' is also accompanied by the upregulation of the intermediate filament protein glial fibrillary acidic protein, the release of growth factors and the formation of the glial scar. However, the signaling cascades which regulate these events, and the molecular mechanisms that give rise to this diverse response, have not been fully elucidated. For example, the role played by degenerating neurons vs. degenerating axons in the activation of astrocytes remains to be determined. To investigate the influence of neuronal cell death vs. axonal degeneration on gliosis, the current study examines the astrocyte response to cochlea removal in two different breeds of adult chickens, one of which exhibits neuronal cell death within the brainstem nucleus magnocellularis (NM) following the lesion and one which does not. Our results indicate that degeneration of NM neurons leads to large increases in both glial proliferation and hypertrophy, while eighth nerve degeneration without NM cell death results in very small increases in glial proliferation.

Alterations in rat striatal glutamate synapses following a lesion of the cortico- and/or nigrostriatal pathway

Ultrastructural changes within the ipsilateral dorsolateral striatum were investigated 1 month following a unilateral ablation of the rat frontal cortex (CTX), removing corticostriatal input, or injection of the neurotoxin, 6-hydroxydopamine (6-OHDA), into the substantia nigra pars compacta, removing nigrostriatal input. In addition, a combined ipsilateral cortical and 6-OHDA lesion (CTX/6-OHDA) was carried out. We find that following a CTX, 6-OHDA, or CTX/6-OHDA lesion, there was a significant decrease in the density of striatal nerve terminal glutamate immunoreactivity compared to the control group. There was also a significant increase in all three lesion groups in the mean percentage of asymmetrical synapses associated with a perforated postsynaptic density. There was a large increase within the CTX/6-OHDA-lesioned group and a smaller but still significant increase in the CTX-lesioned group in the percentage of terminals or boutons with multiple synaptic contacts (i.e., multiple synaptic boutons, MSBs), compared to either the 6-OHDA or the control group. There was no change in any of these measurements within the contralateral striatum. There was a significant decrease in the number of apomorphine-induced contralateral rotations in the CTX/6-OHDA versus the 6-OHDA-lesioned group. Animals receiving just the single CTX or 6-OHDA lesion recovered in motor function compared to the control group as measured by the Rotorod test, while the CTX/6-ODA-lesioned group recovered to less than 50% of the control level. The data suggest that following a CTX and/or 6-OHDA lesion, there is an increase in striatal glutamatergic function. The large increase in the percentage of MSBs in the combined lesion group suggests that dopamine or other factors released by the dopamine terminals assist in regulating synapse formation.

Plasticity of the auditory brainstem: cochleotomy-induced changes of calbindin-D28k expression in the rat

Calbindin is a calcium binding protein that is characteristically expressed in several auditory brainstem nuclei during ontogeny and is thought to serve as a buffer, protecting cells against toxic levels of calcium. Upon maturation, calbindin is drastically reduced or entirely lost in many auditory nuclei. We made cochleotomies in mature rats to study effects of deafening and deafferentation on the expression of calbindin in the auditory brainstem. Following unilateral cochleotomy, we observed a substantial increase in the number of calbindin-immunoreactive fibers and boutons in the ventral subdivisions of the ipsilateral cochlear nucleus. At the same time, calbindin-positive astrocytes emerged in the dorsal and ventral cochlear nucleus. Beyond the immediately affected ipsilateral cochlear nucleus, we found calbindin-positive neurons in the lateral superior olive and in the central inferior colliculus, both contralateral to the operation. The loss of one cochlea reduces auditory input and puts the flow of neuronal activity originating in the two ears out of balance. Our findings indicate that the need for the neuronal networks in the auditory brainstem to adjust to this drastically changed pattern of sensory signals invokes the expression of calbindin in glial cells as well as in directly and indirectly affected neuronal cell populations.

Differential regulation of the growth-associated proteins, GAP-43 and SCG-10, in response to unilateral cortical ablation in adult rats

Synapse replacement after brain injury has been widely documented by anatomical studies in various parts of both the developing and adult nervous system. However, the molecular events that define the specificity of the empirically derived rules of reactive synaptogenesis in different regions of the adult brain remain unclear. In this study we examined the differential regulation of the lesion-induced response of the two growth-associated proteins, superior cervical ganglia-10 and growth-associated protein-43, after unilateral cortex ablation, and determined a hierarchical order for the lesion response from remaining afferent projection neurons originating from the contralateral cortex, ipsilateral thalamus and substantia nigra. We report that in response to unilateral cortex ablation both messenger RNA, by northern blot, and protein, by western blot, for superior cervical ganglia-10 but not growth-associated protein-43 was increased in the homologous area of the contralateral cortex but not the ipsilateral thalamus or substantia nigra. In addition, the specificity of the superior cervical ganglia-10 response, assessed by combined in situ hybridization and retrograde FluoroGold labeling of striatal afferent neurons, found that superior cervical ganglia-10 messenger RNA was increased prominently in layer V pyramidal neurons of the contralateral corticostriatal pathway but was unchanged in afferent projection neurons from the thalamus and substantia nigra. Furthermore, the increase in both superior cervical ganglia-10 messenger RNA and protein seen at three days postlesion in contralateral corticostriatal neurons coincides in time with the initiation of neurite outgrowth in the deafferented striatum by contralateral corticostriatal axons described in our previous ultrastructural study. However, if cortical input to the striatum was removed bilaterally the lesion-induced response for superior cervical ganglia-10 messenger RNA shifted secondarily to thalamostriatal neurons in the ipsilateral thalamus. These data provide evidence that superior cervical ganglia-10 and growth-associated protein-43 are differentially regulated in neurons of the contralateral corticostriatal pathway in response to unilateral cortex ablation and suggests that superior cervical ganglia-10 plays a role in the regulation of neurite outgrowth in the adult striatum after brain injury. However, the specific role that superior cervical ganglia-10 may play in reactive synaptogenesis remains unclear. In addition, our data suggest that a hierarchical order exists for the reinnervation of deafferented striatal neurons after unilateral cortex ablation with preference given to homologous axons from the contralateral cortex.

Glial gene expression during aging in rat striatum and in long-term responses to 6-OHDA lesions

The male rat striatum was examined for age-related changes in mRNAs expressed in astrocytes and microglia in two rat genotypes that differ by 35% in mean and maximum life spans: F344 and the longer-lived F1 (BN x F344) hybrid. The findings extend the established age-related increases in GFAP (glial fibrillary acidic protein) to other glial mRNAs: two lipoprotein mRNAs that are predominantly expressed in striatal astrocytes, apoE (apolipoprotein E) and apoJ (apolipoprotein J, clusterin, CLI, or SGP-2), and two mRNAs expressed in striatal microglia, TGF-beta1 and complement C1qB. By Northern blot hybridization, both genotypes showed progressive increases of GFAP mRNA to > 2.5-fold by the lifespan. Although the rat strains differed 35% in life span, the slope of the GFAP mRNA regression on age did not differ. Relative to GFAP, the increases of apoE, apoJ, C1q, and TGF-beta1 mRNAs were smaller, < or = 1.5-fold. Because prior studies showed that acute damage to striatal afferents induced astrocyte gene expression increases resembling those that also occur during aging, we examined long-term effects of damage to substantia nigra neurons on striatal astrocyte changes during aging. Young F344 rats were given 6-OHDA lesions that cause striatal dopamine deficits and induce GFAP. When examined 15 months later at age 18 months, there was no effect during prior lesions on the age-related elevation of GFAP mRNA. We conclude that aging changes in striatal GFAP mRNAs do not interact with loss of dopaminergic output to the striatum from 6-OHDA lesions and may be independent of the relatively modest dopaminergic losses during normal aging.

Differential uptake of dextran beads by astrocytes, macrophages and oligodendrocytes in mixed glial-cell cultures from brains of neonatal rats

The present study addresses a controversy over the abilities of astrocytes to perform phagocytosis. Primary glial-cell cultures were prepared from the brains of neonatal rats and were incubated with fluorescently-labeled dextran beads (molecular weights approximately 10 and approximately 40 kDa). Astrocytes and oligodendrocytes were double-labeled by immunofluorescence staining of cell-specific markers, and microglia by lectin histochemistry. Cells were permitted to take up beads for 1 h, fixed, and incubated with primary antibodies, followed by fluorescent secondary antibodies or fluorescently-labeled lectin. Macrophages and astrocytes internalized beads of both sizes. In astrocyte processes the beads appeared to line up along glial filaments. The results, which provide direct evidence for uptake of beads by astrocytes in vitro and against equally rapid, if any, uptake by oligodendrocytes, bear upon issues of acid/base balance and glial cell development and are relevant to neuropathological observations in human disease.

Central neuron-glial and glial-glial interactions following axon injury

Axon injury rapidly activates microglial and astroglial cells close to the axotomized neurons. Following motor axon injury, astrocytes upregulate within hour(s) the gap junction protein connexin-43, and within one day glial fibrillary acidic protein (GFAP). Concomitantly, microglial cells proliferate and migrate towards the axotomized neuron perikarya. Analogous responses occur in central termination territories of peripherally injured sensory ganglion cells. The activated microglia express a number of inflammatory and immune mediators. When neuron degeneration occurs, microglia act as phagocytes. This is uncommon after peripheral nerve injury in the adult mammal, however, and the functional implications of the glial cell responses in this situation are unclear. When central axons are injured, the glial cell responses around the affected neuron perikarya appears to be minimal or absent, unless neuron degeneration occurs. Microglia proliferate, and astrocytes upregulate GFAP along central axons undergoing anterograde, Wallerian, degeneration. Although microglia develop into phagocytes, they eliminate the disintegrating myelin very slowly, presumably because they fail to release molecules which facilitate phagocytosis. During later stages of Wallerian degeneration, oligodendrocytes express clusterin, a glycoprotein implicated in several conditions of cell degeneration. A hypothetical scheme for glial cell activation following axon injury is discussed, implying the injured neurons initially interact with adjacent astrocytes. Subsequently, neighbouring resting microglia are activated. These glial reactions are amplified by paracrine and autocrine mechanisms, in which cytokines appear to be important mediators. The specific functional properties of the activated glial cells will determine their influence on neuronal survival, axon regeneration, and synaptic plasticity. The control of the induction and progression of these responses are therefore likely to be critical for the outcome of, for example, neurotrauma, brain ischemia and chronic neurodegenerative diseases.

Lesion-induced axon sprouting in the deafferented striatum of adult rat

Synaptic replacement in rat striatum following a unilateral cortical lesion was investigated using electron microscopy and the anterograde tracer, biotinylated dextrin amine (BDA). In the deafferented striatum evidence of axon sprouting and synapse replacement was seen at 20 days after the lesion and most newly-formed axon terminals were labeled with BDA injected previously into the contralateral cortex. In addition, BDA-labeled fibers from the contralateral cortex formed multiple asymmetric axospinous synapses with deafferented striatal neurons, a morphological feature rarely seen in unlesioned rats. These data suggest that in response to a unilateral cortex lesion axons from the contralateral cortex sprout and reinnervated the deafferented striatal neurons and that reinnervation by 'like' afferents maybe crucial for the establishment of functional recovery after the unilateral cortex lesion.

Differential regulation of astrocytic mRNAs in the rat striatum after lesions of the cortex or substantia nigra

This study evaluates the time course of expression of three astrocytic mRNAs, glial fibrillary acidic protein (GFAP), apolipoprotein E (ApoE), and clusterin, in the rat striatum (ST) following a unilateral lesion of either the cortex (CX) or the substantia nigra (SN), using Northern blot and in situ hybridization analyses. We found that while there was a time-dependent increase in astrocytic GFAP mRNA in the deafferented ST following both the CX and the SN lesions, the time course of the response was different between the two lesion paradigms. Specifically, the increase in GFAP mRNA in striatal astrocytes after the SN lesion was rapid and transient returning to control levels by 10 days postlesion, while the response was long lasting and remained increased until at least 27 days after the CX lesion. In addition, the mRNA response for both ApoE and clusterin was differentially regulated in response to the two lesions. Specifically, both clusterin and ApoE mRNAs were rapidly increased in the ST following the CX lesion while both mRNAs remained unchanged following the SN lesion. Data from this study extend information derived from previous investigations on the multifunctional role of astrocytes in the response to brain injury. Specifically, our data support the notion that while the time course of the GFAP response in striatal astrocytes may vary between lesion paradigms, the upregulation of GFAP is part of a generalized response of reactive astrocytes to diverse brain injuries. By comparison, upregulation of the mRNAs for the lipoproteins clusterin and ApoE are lesion specific and may play a role in the transport of recycled myelin lipids from dying axons to actively growing axons and dendrites in reactive synaptogenesis.

The messenger RNA encoding VGF, a neuronal peptide precursor, is rapidly regulated in the rat central nervous system by neuronal activity, seizure and lesion

The VGF gene encodes a neuronal secretory-peptide precursor that is rapidly induced by neurotrophic growth factors and by depolarization in vitro. VGF expression in the animal peaks during critical periods in the developing peripheral and central nervous systems. To gain insight into the possible functions and regulation of VGF in vivo, we have used in situ hybridization to examine the regulation of VGF messenger RNA by experimental manipulations, and have found it to be regulated in the CNS by paradigms that affect electrical activity and by lesion. Inhibition of retinal electrical activity during the critical period of visual development rapidly repressed VGF messenger RNA in the dorsal lateral geniculate nucleus of the thalamus. In the adult, kainate-induced seizures transiently induced VGF messenger RNA in neurons of the dentate gyrus, hippocampus, and cerebral cortex within hours. Cortical lesion strongly induced VGF messenger RNA in ipsilateral cortex within hours, and strongly repressed expression in ipsilateral striatum. Ten days postlesion there was a delayed induction of VGF messenger RNA in a portion of deafferented striatum where compensatory cortical sprouting has been detected. Expression of the neuronal secretory-peptide precursor VGF is therefore modulated in vivo by monocular deprivation, seizure, and cortical lesion, paradigms which lead to neurotrophin induction, synaptic remodeling and axonal sprouting.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.

Differential spine loss and regrowth of striatal neurons following multiple forms of deafferentation: a Golgi study

Golgi-Cox method and morphometric analyses were used to study the plasticity of striatal medium spiny I neurons in 6-month-old C57BL/6N mice after unilateral or bilateral lesion of the cerebral cortex or combined lesions of the ipsilateral cerebral cortex and intralaminar thalamus. In adult mouse, unilateral lesions of the cerebral cortex did not result in a net gain or loss of linear dendritic length in a randomly selected population of striatal medium spiny I neurons. In addition, there was a well-defined time course of striatal spine loss and replacement occurring after a unilateral cortical lesion. By day 3 postlesion the average 20-microm dendritic segment had lost 30% of the unlesioned control spine value, reached its nadir, lost 45.5%, at 10 days postlesion, and recovered to 80% of unlesioned control levels by 20 days postlesion. The recovery of spines was blocked by a secondary lesion on the contralateral cortex but not on the ipsilateral intralaminar thalamus. These data suggest that striatal medium spiny I neurons of adult mice have a remarkable capacity for plasticity and reactive synaptogenesis following a decortication. The recovery of spine density is primarily induced by axonal sprouting of survival homologous afferent fibers from the contralateral cortex.