Cytotoxic Effect of Brain Macrophages on Developing Neurons

Caudoputamen is damaged by hypocapnia during mechanical ventilation in a rat model of chronic cerebral hypoperfusion

BACKGROUND AND PURPOSE

Postoperative brain dysfunction, such as delirium, is a common complication of anesthesia and is sometimes prolonged, especially in patients with cerebrovascular disease. In the present study we investigated the effect of hypocapnia during anesthesia on neuronal damage using a rat model of chronic cerebral hypoperfusion.

METHODS

Chronic cerebral hypoperfusion was induced by clipping the bilateral common carotid arteries in male Wistar rats. Fourteen days after the operation, these animals were mechanically ventilated for 2 hours and then kept in suitable conditions for an additional 14 days. Twenty-four rats were assigned to 4 groups: those with chronic cerebral hypoperfusion with either hypocapnia or normocapnia during anesthesia, and those given sham operation with either hypocapnia or normocapnia. White matter lesions in the brain sections were evaluated with Klüver-Barrera staining. Proliferation of glial cells was estimated with the use of immunohistochemistry of glial fibrillary acidic protein, a marker for astroglia, and CD11b, a marker for microglia. Computer-assisted morphometry was applied to the immunohistochemical results of microtubule-associated protein 2 to evaluate the loss of neurons.

RESULTS

The histological damage was localized almost exclusively in the white matter in the rats subjected to chronic cerebral hypoperfusion but without hypocapnia. Neuronal damage and astroglial proliferation occurred with aggravated white matter lesions in the caudoputamen in the rats with chronic cerebral hypoperfusion and hypocapnia. No lesions were observed in sham-operated rats with either hypocapnia or normocapnia.

CONCLUSIONS

These results indicate that hypocapnia during anesthesia causes tissue damage in the caudoputamen, which may be responsible for long-lasting postoperative delirium in patients with stroke and/or dementia.

Progress in clinical neurosciences: the evidence for ALS as a multisystems disorder of limited phenotypic expression

Traditionally, amyotrophic lateral sclerosis (ALS) is considered to be a unique neurodegeneration disorder in which motor neurons are selectively vulnerable to a single disease process. Our current understanding of ALS, however, suggests that this is far too limited an approach. While motor neuron degeneration remains the central component to this process, there is considerable phenotypic variability including broad ranges in survivorship and the presence or absence of cognitive impairment. The number of familial variants of ALS for which unique genetic linkage has been identified is increasing, attesting further to the biological heterogeneity of the disorder. At the cellular level, derangements in cytoskeletal protein and glutamate metabolism, mitochondrial function, and in glial interactions are clearly evident. When considered in this fashion, ALS can be justifiably considered a disorder of multiple biological processes sharing in common the degeneration of motor neurons.

Macrophage invasion into injured cochlear nerve and its modification by methylprednisolone

Post-traumatic invasion of macrophages into the cochlear nerve of the rat and measurement of how their invasion was modified by the administration of methylprednisolone were investigated for the first time by using a reproducible and quantifiable experimental model of cochlear nerve injury. Two weeks after precise cochlear nerve compression, a massive invasion of ED1 immunostained macrophages was observed at the compressed portion of the cochlear nerve, and this invasion of macrophages was markedly reduced in the rats to which methylprednisolone had been administered during the pre- and post-compression period. Concomitantly, the residual number of spiral ganglion cells was found to be greater in the compression+methylprednisolone group than in the control compression group. The tissue loss observed in the lesion epicenter was also significantly less in the compression+methylprednisolone group than in the control compression group. The results of our present study demonstrated the effectiveness of methylprednisolone treatment to ameliorate trauma induced cochlear nerve degeneration in the acute phase. However, these results may reflect the sum effects of methylprednisolone on macrophages, including both its beneficial effect by inhibiting the negative aspects of macrophages through attenuating macrophage recruitment to the lesion site, and at the same time an undesirable effect by sacrificing the positive aspects of macrophage function. Moreover, one reservation should be added that the protective effects of steroid to injured cochlear nerve may have operated via a pathway not related to macrophage function. Besides macrophages, various cells and factors participate in the process of CNS injury, and their effects may potentially work either positively or negatively with respect to CNS protection and regeneration at each particular time during the on-going process of CNS injury. Therefore, future investigation in CNS injury should be directed toward understanding such complex mechanisms involved in this process.

Regulation of microglial development: a novel role for thyroid hormone

The postnatal development of rat microglia is marked by an important increase in the number of microglial cells and the growth of their ramified processes. We studied the role of thyroid hormone in microglial development. The distribution and morphology of microglial cells stained with isolectin B4 or monoclonal antibody ED1 were analyzed in cortical and subcortical forebrain regions of developing rats rendered hypothyroid by prenatal and postnatal treatment with methyl-thiouracil. Microglial processes were markedly less abundant in hypothyroid pups than in age-matched normal animals, from postnatal day 4 up to the end of the third postnatal week of life. A delay in process extension and a decrease in the density of microglial cell bodies, as shown by cell counts in the developing cingulate cortex of normal and hypothyroid animals, were responsible for these differences. Conversely, neonatal rat hyperthyroidism, induced by daily injections of 3,5,3'-triiodothyronine (T3), accelerated the extension of microglial processes and increased the density of cortical microglial cell bodies above physiological levels during the first postnatal week of life. Reverse transcription-PCR and immunological analyses indicated that cultured cortical ameboid microglial cells expressed the alpha1 and beta1 isoforms of nuclear thyroid hormone receptors. Consistent with the trophic and morphogenetic effects of thyroid hormone observed in situ, T3 favored the survival of cultured purified microglial cells and the growth of their processes. These results demonstrate that thyroid hormone promotes the growth and morphological differentiation of microglia during development.

Induction of gp91-phox, a component of the phagocyte NADPH oxidase, in microglial cells during central nervous system inflammation

Gp91-phox is an integral component of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex that generates reactive oxygen species (ROS) in activated circulating phagocytes. The authors previously demonstrated that gp91-phox knockout (KO) mice show significant protection from neuronal injury after cerebral ischemia--reperfusion injury, suggesting a pivotal role for this enzyme. Moreover, results from chimeric mice suggested that elimination of gp91-phox from both circulating phagocytes and a putative central nervous system (CNS) source were required to confer neuroprotection. In the current study, the authors demonstrated gp91-phox-specific immunostaining of perivascular cells in the CNS of control rats. However, after transient cerebral ischemia, gp91-phox-positive phagocytes were observed within the core ischemic region and activated microglial cells were positive in the penumbra. Such activated microglial cells were also gp91-phox-positive in the CNS of a chimpanzee with mild meningitis. Finally, in humans, both normal adult CNS tissues and isolated fetal microglial cells expressed gp91-phox mRNA. These microglia also expressed mRNA for the five other known components that comprise the NADPH oxidase complex. These data strongly suggest that microglial cells may contain a functionally active NADPH oxidase capable of generating ROS during CNS inflammation.

Methylprednisolone ameliorates cochlear nerve degeneration following mechanical injury

We investigated whether methylprednisolone sodium succinate can ameliorate cochlear nerve degeneration following compression injury on the cerebellopontine angle portion of the cochlear nerve, using a quantitative animal experimental model that we have developed recently. In this model, cochlear nerve degeneration after compression could be quantitatively evaluated, while cochlear ischemia induced by the compression carefully maintained below the critical limit that causes irreversible damage to the cochlea. Eleven rats were treated with methylprednisolone during the pre- and post-compression period. Two weeks after compression, the numbers of SGC were compared between the rats that received the compression without and with methylprednisolone treatment. Methylprednisolone treatment improved the survival of SGC following cochlear nerve injury statistically highly significantly in the basal turn where the traumatic stress had been less than in the other cochlear turns in our experimental setting. Although it was not statistically significant, greater survival was also observed in the other cochlear turns. The results of this experimental study indicated that at least a portion of injured cochlear nerve had been potentially treatable, and that methylprednisolone might prevent such cochlear neurons from entering into the vicious process of irreversible damaging process.

Microglial contribution to oxidative stress in Alzheimer's disease

Microglia are the CNS macrophage and are a primary cellular component of plaques in Alzheimer's disease (AD) that may contribute to the oxidative stress associated with chronic neurodegeneration. We now report that superoxide anion production in microglia or macrophages from 3 different species is increased by long term exposure (24 hours) to A beta peptides. Since A beta competes for the uptake of opsonized latex beads and for the production of superoxide anion by opsonized zymosan, a likely site of action are membrane receptors associated with the uptake of opsonized particles or fibers. The neurotoxic fibrillar peptides A beta (1-42) and human amylin increase radical production whereas a non-toxic, non-fibrillar peptide, rat amylin, does not. We also report that the effect of A beta peptides on superoxide anion production is not associated with a concomitant increase in nitric oxide (NO) production in either human monocyte derived macrophages (MDM) or hamster microglia from primary cultures. Since NO is known to protect membrane lipids and scavenge superoxide anion, the lack of A beta-mediated induction of NO production in human microglia and macrophages may be as deleterious as the over-production of superoxide anion induced by chronic exposure to A beta peptides.

Endogenous and exogenous fibroblast growth factor 2 support survival of chick retinal neurons by control of neuronal neuronal bcl-x(L) and bcl-2 expression through a fibroblast berowth factor receptor 1- and ERK-dependent pathway

Fibroblast growth factor (FGF) 2 is a survival factor for various cell types, including retinal neurons. However, little is understood about the molecular bases of the neuroprotective role of FGF2 in the retina. In this report, FGF2 survival activity was studied in chick retinal neurons subjected to apoptosis by serum deprivation. Exogenous FGF2 supported neuronal survival after serum deprivation and increased neuronal bcl-x(L) and bcl-2 expression, through binding to its receptor R1 (FGF-R1), and subsequent extracellular signal-regulated kinase (ERK) activation. Endogenous FGF2 was transiently overexpressed after serum deprivation. Its down-regulation by antisense oligonucleotides and blockade of its signaling pathway (binding to FGF-R1, tyrosine phosphorylation, and ERK inhibition) decreased bcl-x(L) and bcl-2 levels and and enhanced apoptosis, suggesting that endogenous FGF2 supported neuronal survival through a pathway similar to that of exogenous FGF2. This pathway may serve to up-regulate, or maintain, bcl-x(L) and bcl-2 levels that normally decrease during the onset of apoptosis. Indeed, long-term ERK activation and high bcl-x(L) levels are necessary for the survival activity of both exogenous and endogenous FGF2. Because FGF2 is upregulated following retinal injury in vivo, we suggest that an injury-stimulated autocrine/paracrine FGF2 loop may serve to maintain high levels of survival proteins, such as Bcl-x(L), through ERK activation in retinal neurons.

LPS-induced neuroinflammatory effects do not recover with time

Inflammatory processes develop in the vicinity of the neuropathological hallmarks associated with Alzheimer's disease (AD) and may play a role in the progression of the disease and its clinical expression. We have previously reported that chronic infusion of LPS into the fourth ventricle of rat brains reproduced many of the inflammatory and pathological changes seen in the brain of AD patients. In the current study, we used the same animal model to investigate the effects of longer infusion of LPS and whether these effects could recover over time. The results show that doubling the time of LPS infusion did not increase the inflammatory reaction and did not produce a significantly greater behavioral impairment. Waiting for 37 days after the cessation of the LPS infusion did not decrease the density of activated microglia and did not improve performances in the Morris water maze task. The results suggest that inflammation may contribute to the pathogenic mechanisms that underlie the clinical expression of AD.

GDNF and NT-4 protect midbrain dopaminergic neurons from toxic damage by iron and nitric oxide

Free radical formation is considered to be a major cause of dopaminergic (DAergic) cell death in the substantia nigra leading to Parkinson's disease (PD). In this study we employed several radical donors including iron and sodium nitroprusside to induce toxic effects on DAergic neurons cultured from the embryonic rat midbrain floor. Overall cell survival was assessed by assaying LDH, and DAergic neuron survival was monitored by counting tyrosine hydroxylase-positive cells. Our data suggest that the DAergic neuron population is about fourfold more susceptible to free-radical-mediated damage than the total population of midbrain neurons. Application of the neurotrophic factors GDNF and NT-4, for which DAergic neurons have specific receptors, prior to toxin administration protected these neurons from toxin-mediated death, which, fully or in part, occurs under the signs of apoptosis. These findings underscore the importance of GDNF and NT-4 in designing future therapeutical concepts for PD.

Role of fibroblast growth factor-2 in human brain: a focus on development

Trophic factors have gained a great degree of attention as regulators of neural cells proliferation and differentiation as well as of brain maturation. Very little is known, however, about their effects on human immature nervous system. In this paper, data on expression of fibroblast-growth factor-2 and its receptors are reviewed and discussed in the light of its possible role in human brain development.

A morphological analysis of the motor neuron degeneration and microglial reaction in acute and chronic in vivo aluminum chloride neurotoxicity

The monthly intracisternal inoculation of aluminum chloride (AlCl3) to young adult New Zealand white rabbits induces motor neuron degeneration marked by intraneuronal neurofilamentous aggregates similar to that observed in amyotrophic lateral sclerosis (ALS). However, in contrast to ALS, this process occurs in the experimental paradigm in the absence of a glial response. In addition, whereas ALS is a fatal disorder, the cessation of aluminum exposure leads to both clinical and neuropathological recovery. Because microglia can influence neuronal regeneration, we have examined the effect of both acute and chronic aluminum exposure on microglial activation in vivo. We have studied microglial morphology in young adult New Zealand white rabbits receiving either single (1000 microg) or repeated sublethal (100 microg monthly) intracisternal inoculums of AlCl3. In addition, rabbits receiving 1000 microg AlCl3 inoculums were studied following an unilateral sciatic axotomy 48 h prior to the AlCl3 exposure. Our studies demonstrate that microglial activation in vivo is inhibited by AlCl3 exposure, and that a correlation exists between the extent of microglia suppression and the potential for recovery. This suggests that microglial activation is an important determinant of neuronal injury.

Molecular signals for glial activation: pro- and anti-inflammatory cytokines in the injured brain

Injury to the central nervous system leads to cellular changes not only in the affected neurons but also in adjacent glial cells. This neuroglial activation is a consistent feature in almost all forms of brain pathology and appears to reflect an evolutionarily-conserved program which plays an important role for the repair of the injured nervous system. Recent work in mice that are genetically-deficient for different cytokines (M-CSF, IL-6, TNF-alpha, TGF-beta 1) has begun to shed light on the molecular signals that regulate this cellular response. Here, the availability of cytokine-deficient animals with reduced or abolished neuroglial activation provides a direct approach to determine the function of the different components of the cellular response leading to repair and regeneration following neural trauma.

Estrogen and microglia: A regulatory system that affects the brain

Sex hormones are involved in the physiological regulation of several aspects of behavior and neuroendocrine events. It has been accepted that such effects are mediated directly by steroid actions on neurons; however, new studies have shown that the glial cells are also affected by gonadal steroids. The microglia are one specialized brain glial cell type, which is a target for estrogen actions. In fact, we believe that many of the immune and nonimmune regulatory functions of microglia in the brain are influenced directly by estrogen via expression and secretion of cytokines, and growth factors by the microglia. The present review details only a section of the known aspects of microglial function, focusing mainly on nonimmune regulatory actions in the brain and their functional relationship with sex hormones. Moreover, we present evidence for the presence of estrogen receptor-beta (ERbeta) in rat microglial cells.

Pyruvate and lactate protect striatal neurons against N-methyl-D-aspartate-induced neurotoxicity

A sustained release of glutamate contributes to neuronal loss during cerebral ischaemia. Using cultured mouse striatal neurons, we observed that glucose deprivation, which occurs in this pathological process, enhanced the N-Methyl-D-aspartate (NMDA)- or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-induced neurotoxicity. The end products of glycolysis, lactate and pyruvate, strongly protected neurons from these neurotoxic effects. The neuroprotective effect of pyruvate (which is more prominent in the absence of glucose) was not related to its ability to react with H2O2 by a decarboxylation process. Pyruvate and L-lactate strongly counteracted the deep decrease in the neuronal ATP content induced by NMDA, indicating that they might protect striatal neurons by rescuing cellular energy charge. Addition of MK-801 after the NMDA withdrawal completely protected neurons, suggesting that NMDA neurotoxicity resulted from a delayed NMDA receptor activation probably linked to a delayed release of an endogenous agonist in the extracellular medium. The strong accumulation of extracellular glutamate which was found in both sham and NMDA-treated cultures was markedly decreased by pyruvate. Thus, pyruvate might also exert its protecting activity by decreasing the delayed accumulation of glutamate which seemed to be neurotoxic only after a preexposure of neurons to NMDA.

The novel pyrrolopyrimidine PNU-101033-E improves facial motor neuron survival following intracranial axotomy of the facial nerve in the adult rat

Neuronal survival is important to functional restitution following axotomy. Proximal lesions of the facial nerve, due to head trauma or tumor growth, for example, may cause long-standing or even permanent facial nerve palsy. Betamethasone has been used by several neurosurgical clinics for the treatment of postoperative facial nerve palsy; however, this practice is based only on clinical experience. The aim of the present study was to explore the putative effect on facial motor neuron survival of a novel lazaroid (pyrrolopyrimidine, PNU-101033-E) and furthermore to compare the effects with those of betamethasone, following intracranial transection of the facial nerve in adult rats. Both agents are known to inhibit lipid peroxidation by free radical scavenging. The lesion model used has recently been reported to induce massive neuronal cell death with a relative survival of 26.8 +/- 11.3% 1 month after lesion. Oral administration of lazaroids or daily injections of betamethasone followed surgery for 1 month, after which quantification of motor neuronal profiles was performed in the facial nucleus. Lazaroid-treated animals showed a significantly enhanced neuronal survival (68.0 +/- 9.8%), whereas no significant difference was found in betamethasone-treated animals (33.1 +/- 11.7%). The microglial and astrocytic responses in the facial nucleus were intense on the operated sides in betamethasone-treated as well as lazaroid-treated animals, and no differences in comparison with untreated animals were found. In conclusion, we found that the novel pyrrolopyrimidine PNU-101033-E, but not betamethasone, significantly enhanced nerve cell survival. This agent may therefore serve as a useful neuroprotective agent following intracranial trauma to the facial nerve and should be further evaluated for clinical use.

Microinjection of catalase cDNA prevents hydrogen peroxide-induced motoneuron death

Oxidative stress is believed to play a central role in the pathogenesis of amyotrophic lateral sclerosis (ALS). We investigated the protective effects of overexpression of catalase in primary cultures of rat spinal motoneurons against the oxidative stress of hydrogen peroxide. Using microinjection, catalase-encoding cDNA was transferred into the motoneurons. In another approach, motoneurons were injected with a catalase solution. Both procedures elevated the intracellular antioxidant status of the cultured motoneurons as evidenced by a significant protection against H2O2 toxicity. We conclude that modulating the expression of enzymes involved in cellular defense against oxidative stress can render cells more resistant to oxidant toxicity.

Extensive neuronal cell death following intracranial transection of the facial nerve in the adult rat

The aim of the present study is to examine the neuronal degeneration and the glial response following intracranial transection of the facial nerve close to the brainstem and furthermore to compare the results with a distal nerve injury. The facial nerve was cut either intracranially in the posterior cranial fossa or further distally, where it passes the parotid gland, in adult rats. Intracranial axotomy caused a massive loss of neuronal profiles. Only 26.8+/-11.3% of facial motor neuronal profiles were found ipsilateral to the nerve injury when compared to the contralateral side, following intracranial axotomy. This was statistically significant in comparison to the distal injury (72.4+/-9.5%), 4 weeks post-lesion. Reactive microglial cells expressed ED1 immunoreactivity following the intracranial axotomy but not following the distal nerve injury. In conclusion, there was a large discrepancy in neuronal degeneration as well as presence of phagocytic (ED1 positive) microglia between the two lesions. The intracranial lesion model used in the present study generates a massive neuronal cell death and should therefore be a useful tool for studies on proximal cranial nerve injuries and in particular mechanisms causing cell death, which may occur following, for example, head trauma.

Neuroglial activation repertoire in the injured brain: graded response, molecular mechanisms and cues to physiological function

Damage to the central nervous system (CNS) leads to cellular changes not only in the affected neurons but also in adjacent glial cells and endothelia, and frequently, to a recruitment of cells of the immune system. These cellular changes form a graded response which is a consistent feature in almost all forms of brain pathology. It appears to reflect an evolutionarily conserved program which plays an important role in the protection against infectious pathogens and the repair of the injured nervous system. Moreover, recent work in mice that are genetically deficient for different cytokines (MCSF, IL1, IL6, TNFalpha, TGFbeta1) has begun to shed light on the molecular signals that regulate this cellular response. Here we will review this work and the insights it provides about the biological function of the neuroglial activation in the injured brain.

Requirement for nitric oxide in retinal neuronal cell death induced by activated Müller glial cells

Retinal Müller glial cells express the inducible isoform (-2) of nitric oxide (NO) synthase (NOS) in vitro after stimulation by lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) or in vivo in some retinal pathologies. Because NO may have beneficial or detrimental effects in the retina, we have used cocultures of retinal neurons with retinal Müller glial (RMG) cells from mice disrupted for the gene of NOS-2 [NOS-2 (-/-)] to clarify the role of NO in retinal neurotoxicity. We first demonstrated that NO produced by activated RMG cells was not toxic for RMG cells themselves. Second, the NO released from LPS/IFN-gamma-stimulated RMG cells induced neuronal cell death, because no neuronal cell death has been observed in cocultures with RMG cells from NOS-2 (-/-) mice and because inhibition of NOS-2 induction by transforming growth factor-beta or blockade of NO release by different NOS inhibitors prevented neuronal cell death. Addition of urate, a peroxynitrite scavenger, or superoxide dismutase partially prevented neuronal cell death induced by NO, whereas the presence of a poly(ADP-ribose) synthetase inhibitor, caspase inhibitors, or a guanylate cyclase inhibitor had no significant effect on cell death. These results demonstrated that a large release of NO from RMG cells is responsible for retinal neuronal cell death in vitro, suggesting a neurotoxic role for NO and peroxynitrite during retinal inflammatory or degenerative diseases, where RMG cells were activated.

A cyclooxygenase-2 inhibitor attenuates white matter damage in chronic cerebral ischemia

The effects of nimesulide, a cyclooxygenase-2 inhibitor, were examined during chronic cerebral hypoperfusion. After bilateral ligation of the common carotid arteries in 30 rats, 21 received dosages of 2 or 5 mg/kg nimesulide daily and nine received vehicle daily for 14 days. The serum was then analyzed biochemically, and pathological changes were estimated in the white matter by the emergence of major histocompatibility complex (MHC) antigen-immunoreactive activated microglia and white matter lesions. In the vehicle-treated animals, activated microglia and white matter lesions were observed. Following treatment with either 2 or 5mg/kg nimesulide, the magnitude of these changes was reduced (p < 0.001) without significant side effects. These results indicate a potential use for cyclooxygenase-2 inhibitors in cerebrovascular disease.

The effect of microglia on embryonic dopaminergic neuronal survival in vitro: diffusible signals from neurons and glia change microglia from neurotoxic to neuroprotective

When embryonic dopaminergic neurons are transplanted into the adult brain, approximately 95% die within a few days. To assess whether microglia activated during transplantation might be responsible for this rapid death, we examined the effect of microglia on rat embryonic dopaminergic neurons in vitro. Conditioned medium from 7-day-old microglia was found to decrease the number of dopamine neurons surviving in primary culture, but activation of the microglia with N-formyl-methionyl-leucyl-phenylalanine (FMLP) or Zymosan A did not increase the toxicity of the conditioned medium. We next tested the effect of coculturing microglia and dopaminergic neurons by placing microglia in semipermeable well inserts over the neuronal cultures. The presence of microglia now increased dopaminergic neuronal survival, microglial activation again having no effect. To increase yet further the possible interactions between microglia and neurons, the mesencephalic cells and microglia were mixed together and placed as a tissue in three-dimensional culture, and here again the presence of microglia increased dopaminergic neuronal survival with no effect of activation. Contact of microglia with the mesencephalic cells therefore converted them from being toxic to dopaminergic neurons to promoting their survival. The change in microglial effect from toxic to protective was caused by soluble molecules secreted by cells in the neuronal cultures, as conditioned medium derived from microglia-neuronal cocultures also had a dopaminergic neuron survival effect, indicating that microglia in cocultures behave differently from microglia removed from neuronal and glial influence. Microglia cocultured with either neurons or astrocytes downregulated inducible nitric oxide synthase (iNOS), indicating a decrease in the production of nitric oxide and possibly other toxic molecules. These findings indicate that in their natural environment, microglia are likely to be beneficial for the survival of embryonic dopaminergic grafts.

Expression of platelet-derived growth factor B-chain and beta-receptor in hypoxic/ischemic encephalopathy of neonatal rats

Expression of platelet-derived growth factor B-chain and of its specific receptor (beta-receptor) was investigated in immature brains with hypoxic/ischemic injury. After the left common carotid arteries of seven-day-old rats were ligated and pups were placed in a hypoxic chamber, the protein and messenger RNA of both B-chain and beta-receptor were assessed using immunocytochemistry and northern analysis, respectively. Transcripts for B-chain were localized by in situ hybridization. Faint but definite expression of B-chain and beta-receptor was seen in the brains of untreated neonatal controls. Three to 48 h after hypoxia B-chain protein was generally increased above control levels, but focally decreased expression was seen in infarcted areas. Enhanced induction of messenger RNA of B-chain was seen in the both sides of cerebral cortices and hippocampi at 3 h. Strongly increased positivity for B-chain protein and mRNA occurred in the neurons surrounding the infarct. In situ hybridization still showed this up-regulation seven days after hypoxia. Beta-receptor protein expression was enhanced in some neurons immediately surrounding the infarct at 3 h of hypoxia, and marked up-regulation was seen at 16 h. Beta-receptor messenger RNA remained at control levels. Immunocytochemistry showed strong immunoreactivity for the beta-receptor on the neurons surrounding the infarct at 72 h. These results indicate that a neonatal hypoxic/ischemic insult induces neuronal up-regulation of the platelet-derived growth factor B-chain as well as beta-receptor immediately after hypoxia. While this up-regulation is relatively transient in most neurons, sublethal damage to neurons immediately surrounding an infarct induces sustained up-regulation. Through autocrine and paracrine mechanisms, platelet-derived growth factor B-chain molecules may act as a neuroprotective factor in immature brain experiencing with hypoxic/ischemic injury.

Propentofylline protects neurons in culture from death triggered by macrophage or microglial secretory products

We recently demonstrated that conditioned medium (CM) from peritoneal macrophages or activated microglia triggers a predominantly apoptotic death in hippocampal neurons in culture. We tested the effects of propentofylline (ppf), an agent that is neuroprotective in focal ischemia and is also associated with reduced microglial antigen expression after insult. Ppf had no impact on the secretion of neurotoxin from microglia. However, ppf significantly attenuated the effects of macrophage and microglial conditioned medium on neurons. Ppf did not attenuate neuronal hypoxic injury but did reverse the exaggeration of hypoxic injury exerted by subsequent addition of macrophage CM. A1 and A2 adenosine receptor inhibitors and an inhibitor of adenosine uptake each mimicked the effect of ppf. Neither ATP nor a deaminase inhibitor blocked the effect of microglial CM. These findings may be relevant to the neuroprotective effects of ppf in ischemia and dementia.

Nimodipine-induced improved survival rate of facial motor neurons following intracranial transection of the facial nerve in the adult rat

OBJECT

Neuronal survival is an important factor in the achievement of functional restitution after peripheral nerve injuries. Intracranial tumors or trauma may cause patients to exhibit a temporary or permanent facial nerve palsy. Nimodipine, which acts as an antagonist to L-type voltage-gated calcium channels, has been shown to be neuroprotective in various lesion models of the central and peripheral nervous systems. The aim of the present study was to evaluate the effect of nimodipine on motor neuron survival in the facial motor nucleus following intracranial transection of the adult rat facial nerve.

METHODS

The facial nerve was cut intracranially in the posterior cranial fossa. Nimodipine was administered orally preoperatively for 3 days and postoperatively for up to 1 month, after which the number of neuronal profiles was quantified. The glial reaction was studied in the facial nucleus for up to 1 month by using immunocytochemical analysis. There was a significantly larger proportion of surviving motor neurons 1 month postinjury in animals treated with nimodipine (61+/-6.7%) in comparison with untreated animals (26.8+/-11.3%). Immunocytochemical analysis showed an increase in the amount of OX42 (microglia), ED1 (macrophages), and anti-glial fibrillary acidic protein (astrocytes) ipsilateral to the nerve injury; however, there was no difference between the two experimental groups of animals 2 to 28 days after surgery.

CONCLUSIONS

The authors propose a neuroprotective role for nimodipine, which may be useful as a "cranial nerve protective agent" following insults such as head injury or skull base surgery.

Neuronal death and survival in two models of hypoxic-ischemic brain damage

Two unilateral hypoxic-ischemia (HI) models (moderate and severe) in immature rat brain have been used to investigate the role of various transcription factors and related proteins in delayed neuronal death and survival. The moderate HI model results in an apoptotic-like neuronal death in selectively vulnerable regions of the brain while the more severe HI injury consistently produces widespread necrosis resulting in infarction, with some necrosis resistant cell populations showing evidence of an apoptotic type death. In susceptible regions undergoing an apoptotic-like death there was not only a prolonged induction of the immediate early genes, c-jun, c-fos and nur77, but also of possible target genes amyloid precursor protein (APP751) and CPP32. In contrast, increased levels of BDNF, phosphorylated CREB and PGHS-2 were found in cells resistant to the moderate HI insult suggesting that these proteins either alone or in combination may be of importance in the process of neuroprotection. An additional feature of both the moderate and severe brain insults was the rapid activation and/or proliferation of glial cells (microglia and astrocytes) in and around the site of damage. The glial response following HI was associated with an upregulation of both the CCAAT-enhancer binding protein alpha (microglia only) and NFkappaB transcription factors.

Propentofylline attenuates microglial reaction in the rat spinal cord induced by middle cerebral artery occlusion

This study examines the effect of Propentofylline (PPF) on reactive microglia in the lumbar spinal cord in rats following focal cerebral ischaemia produced by permanent occlusion of the middle cerebral artery (MCA). Our results showed that daily treatment of PPF beginning at 24 h after MCA occlusion for 2 or 4 consecutive days markedly suppressed the microglial response as detected immunohistochemically with OX-42. The most dramatic effect was the prevention of transformation of ramified microglia into amoeboidic form as well as formation of perineuronal microglia in close association with the soma of motoneurons. This has greatly amplified the potentiality of PPF used as a neuroprotective drug against microglia-related neuron damage induced by cerebral ischaemia.

Complement activation and CD59 expression in the motor facial nucleus following intracranial transection of the facial nerve in the adult rat

Intracranial transection of the facial nerve has been shown to cause a massive neuronal cell death in the motor facial nucleus. Complement activation has been proposed to contribute to neuronal degeneration following axotomy. Using immunocytochemistry and in situ hybridization we show in the present study that there is complement activation in the facial nucleus after intracranial facial nerve transection as well as increase of the complement regulators CD59 and clusterin. We propose a neuroprotective role for the complement regulators CD59 and clusterin against homologous attack of complement to facial motor neurons.

Morphological, biochemical and behavioural changes induced by neurotoxic and inflammatory insults to the nucleus basalis

Interest in the basal forebrain cholinergic system has greatly increased since neuropathological studies in humans provided evidence that this system is severely affected in Alzheimer's disease and other dementing disorders. In laboratory animals, disruption of the nucleus basalis cholinergic neurones has been produced by several neurotoxic insults in order to obtain a model reproducing the behavioural impairment related to the cholinergic deficits. The experiments reported in this review demonstrate that excitotoxic amino acids, beta-amyloid and lipopolysaccharide, injected directly in the nucleus basalis are toxic to the cholinergic neurones in the rat. The excitotoxin lesions of the nucleus basalis, although not selective, are a useful tool for producing experimental animals with cholinergic hypofunction and for investigating drugs able to ameliorate the cholinergic functions. Local injections of amyloid peptides in the rat nucleus basalis produced cholinergic hypofunction and some behavioural impairment. Finally, an intense glia reaction with a limited cholinergic hypofunction and no behavioural impairment was induced by a 4-week infusion of lipopolysaccharide in the nucleus basalis. In conclusion, all three models, in spite of their limitations, offer useful tools for the study of the pathogenetic mechanisms of Alzheimer's disease and for investigating potentially useful drugs.

Identification of CSF-1 as a brain macrophage migratory activity produced by astrocytes

Intraparenchymal migration of macrophages occurs in the CNS during development or as a consequence of tissue injuries. In the present study, we have shown, by using an in vitro chemotaxis assay, that cultured rat astrocytes obtained from the developing cerebral cortex and striatum produce soluble factors, which attract purified brain macrophages. The effect of astrocyte-derived factors on macrophages was strongly reduced in the presence of antibodies neutralizing colony-stimulating factor 1 (CSF-1, also called M-CSF), and recombinant CSF-1 was found to act as a chemotactic agent on brain macrophages. Synthesis of CSF-1 by cultured astrocytes was confirmed by northern detection of CSF-1 transcripts. In contrast, the CSF-1 gene was not expressed by cultured neurons from the cerebral cortex and striatum or by the brain macrophage population responsive to CSF-1 gradient. ELISA detection of CSF-1 in tissue extracts revealed the occurrence of this cytokine in the rat cerebral cortex during postnatal development and in adults. Altogether, our results demonstrate that astrocytes, through CSF-1 secretion, can trigger the polarized migration of brain macrophages and suggest a new mechanism which could regulate the locomotion of these cells in the cerebral cortex during ontogenesis or following lesions.

Lipopolysaccharide differentially regulates microglial trk receptor and neurotrophin expression

Activated brain microglia play a pivotal role in inflammatory and degenerative disorders, mediating immune function and producing toxic and trophic agents. We previously reported that microglia express neurotrophins and that neurotrophin-3 (NT-3) increases microglial proliferation and phagocytosis, processes associated with cellular activation. However, mechanisms regulating responsiveness to NT-3 and expression of NT-3 in activated microglia remain undefined. To investigate mechanisms governing microglial responsiveness to neurotrophins, we determined whether microglia express trk C, the high-affinity receptor for NT-3, and whether the inflammatory agent lipopolysaccharide (LPS) regulates receptor expression. Trk C mRNA was expressed by unstimulated microglia, and both trk C mRNA and protein were dramatically increased by LPS. In contrast, expression of trk A, the high-affinity receptor for nerve growth factor (NGF), was down-regulated by LPS. Consequently, the same stimulus differentially influences responsiveness of microglia to distinct trophins. In addition, LPS induced microglial NT-3 expression, suggesting that increases in both the ligand and receptor modulate NT-3 effects on microglia. Regulation was specific, since brain-derived neurotrophic factor (BDNF) and NT-4/5 expression were unaltered by LPS. In sum, our findings raise the possibility that microglial NT-3 regulates their response to inflammation through autocrine mechanisms: LPS modulates both trk C and NT-3 which, in turn, regulate microglial function.

Upregulation of a new microglial gene, mrf-1, in response to programmed neuronal cell death and degeneration

Cerebellar granule neurons isolated from postnatal day 7 (P7) rats and grown in normal K+ medium begin to degenerate at approximately 4 d in vitro (DIV) and die. To search for genes upregulated in the process of neuronal cell death, differential hybridization was performed with subtracted cDNA probes and a cDNA library from 5 DIV. One of the genes isolated was microglial response factor-1 (mrf-1), which encoded a sequence of 177 amino acids with a single EF-hand calcium-binding motif. By Northern blots, the transcript was upregulated in cerebellar culture at 4 DIV, peaked at 6 DIV, and decreased at 7 DIV. Upregulation was also found when the apoptosis of granule cells was induced by replacing high K+ medium with normal K+ medium. However, when non-neuronal cells were thoroughly eliminated with aphidicolin, an antimitotic agent, the upregulation at 4-7 DIV did not occur. By immunocytochemistry, MRF-1 was detected at 5 DIV in OX-42-positive cells (microglia), and it exhibited an increase in response to granule cell death. MRF-1 levels in microglia purified from cerebral cortex also upregulated in the presence of 5 DIV granule cells. In the developing cerebellum in vivo, levels of mrf-1 mRNA transiently increased in the early postnatal stages, reaching a peak at P7 when cerebellar neurons and astrocytes undergo extensive apoptosis. In adult brain sections, MRF-1 was detected in the perikarya and processes of ramified/resting microglia, and peripheral motor nerve dissection prominently increased the expression in activated microglia surrounding injured central motoneurons. Therefore, mrf-1 appears to be one of the microglial genes that respond to neuronal cell death and degeneration.

Reoxygenation increases the release of reactive oxygen intermediates in murine microglia

Respiratory burst activity of murine microglial cells was investigated in vitro under normoxic and hypoxic conditions with a chemoluminometric assay. Hypoxia for 24 hours reduced the release of extracellular reactive oxygen intermediates (ROIs), whereas reoxygenation increased the chemoluminescence more than sevenfold. Blockade of potassium channels inhibited the increase of oxidative burst after reoxygenation, indicating that potassium ions, which were increased in the supernatant of hypoxic microglial cells, were involved in this activation process. Also, blockade of voltage-gated calcium channels with nifedipine attenuated the increased release of ROIs. With fura-2 analysis, it was shown that the activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase by potassium ions was mediated by calcium influx via voltage-gated calcium channels. Thus, influx of calcium ions through voltage-gated channels activates the NADPH oxidase in microglial cells during reoxygenation. By the increased production of ROIs, microglial cells may add to the reperfusion injury after ischemia in vivo.

Measurement and characterization of superoxide generation in microglial cells: evidence for an NADPH oxidase-dependent pathway

While oxygen free radicals are important mediators of brain injury, questions remain regarding which cell types and enzyme pathways trigger this radical generation. Microglial cells have been hypothesized to be an important source of radical generation; however, the magnitude, kinetics, and mechanism of this process are unknown. Oxygen radical generation by stimulated primary microglia was directly measured and characterized by electron paramagnetic resonance spin trapping. Microglia, when stimulated by phorbol ester or opsonified zymosan, gave rise to EPR spectra characteristic of superoxide. Experiments performed in the presence of superoxide dismutase, catalase, deferoxamine, and dimethyl sulfoxide excluded generation of hydroxyl radicals in significant amounts. Microglial superoxide generation was blocked by the NADPH oxidase inhibitor diphenylene iodonium in a manner similar to that seen in neutrophils, suggesting that a neutrophil like NADPH oxidase was the source of superoxide production. However, microglia produced 20 to 40 times less superoxide compared to a similar number of neutrophils during the first 30 min following stimulation, indicating a marked difference in the regulation of NADPH oxidase activation. Western blots of microglia lysates demonstrated that both large (gp91-phox) and small (p22-phox) NADPH oxidase subunits are expressed in both unstimulated and stimulated microglia. Indirect immunofluorescence demonstrated localization at the membrane surfaces of activated cells. Thus, microglial cells generate superoxide via a neutrophil-like NADPH oxidase but exhibit distinctly different time course and magnitude of activation than that seen in neutrophils.

Dose-dependent, protective effect of FK506 against white matter changes in the rat brain after chronic cerebral ischemia

Neuroprotective effects of immunosuppressive agents have been shown in cerebral ischemia. To investigate the role of immunosuppressive agents in chronic cerebral ischemia and to design a drug protocol with safe therapeutic windows, we examined the effects of FK506, a potent immunosuppressive agent, on chronic cerebral ischemia. Both common carotid arteries were ligated in 73 male Wistar rats. Fifty-eight of these rats received a chronic injection of FK506 (0.2, 0.5, 1.0 mg/kg) and the remaining 15 received a vehicle solution injection. Microglia/macrophage was investigated with immunohistochemistry for leukocyte common antigen and major histocompatibility complex, and astroglia was examined with glial fibrillary acidic protein as markers. White matter rarefaction and the number of immunopositive glial cells were assessed from 7 to 30 days after the ligation. In the vehicle-treated animals, there was persistent and extensive activation of the microglia/macrophages and astroglia in the white matter, including the optic nerve, optic tract, corpus callosum, internal capsule, anterior commissure and traversing fiber bundles of the caudoputamen. In the FK506-treated rats, the number of activated microglia/macrophages was significantly reduced in a dose-dependent manner (p<0.01) as compared to the vehicle-treated rats. Rarefaction of the white matter was also inhibited by FK506 in a dose-dependent manner (p<0. 01). Thus, a clinically-relevant dosage of FK506 attenuated both glial activation and white matter changes in chronic cerebral ischemia in the rat. These results indicate a potential use for FK506 in cerebrovascular diseases.

Gene expression in activated brain microglia: identification of a proteinase inhibitor that increases microglial cell number

Microglia, the intrinsic immune cells of the central nervous system, are activated in a variety of inflammatory brain diseases in which they play a pathogenetic role. However, mechanisms underlying activation are largely unknown. To begin elucidating molecular mechanisms associated with activation, we characterized the pattern of gene expression in virtually pure dissociated microglial cultures, using RT-PCR differential display. Microglia were activated with bacterial lipopolysaccharide (LPS), a traditional stimulant, and the profile of gene expression was compared to that in basal, control cultures. Activation resulted in altered expression of six genes. The cDNAs were isolated, sequenced and characterized. Homology searches identified three novel genes, and two that exhibited very high sequence similarity to the gene encoding squamous cell carcinoma antigen (SCCA). SCCA (1 and 2) are tandemly arranged genes that encode two serine proteinase inhibitors (serpins). SCCA has been detected exclusively in cancer cells, and is a plasma marker for squamous cell carcinoma. Immunoblot analysis indicated that gene expression was accompanied by a 5-fold increase in the synthesis of SCCA protein in LPS-activated microglia. To assess potential biological actions of the SCCA serpins, SCCA1 protein was added to cultures. SCCA1 altered microglial morphology, and elicited a dramatic, 5-fold increase in cell number within 72 h. The effects appeared to be cell-specific, since the protein had no effect on other cell types: cortical astrocytes and neurons from cortex or basal forebrain were unaffected. We tentatively conclude that SCCA1 may play a cell-specific role in increasing cell number, a critical early step in microglial activation and brain inflammation. More generally, differential display of genes in the microglial model system may help define patterns of expression associated with CNS disease, thereby identifying pathogenetic mechanisms and new therapeutic targets.

Glutamate neurotoxicity is associated with nitric oxide-mediated mitochondrial dysfunction and glutathione depletion

The role of mitochondrial energy metabolism in glutamate mediated neurotoxicity was studied in rat neurones in primary culture. A brief (15 min) exposure of the neurones to glutamate caused a dose-dependent (0.01-1 mM) increase in cyclic GMP levels together with delayed (24 h) neurotoxicity and ATP depletion. These effects were prevented by either the nitric oxide (.NO) synthase (NOS) inhibitor Nomega-nitro-L-arginine methyl ester (NAME; 1 mM) or by the N-methyl-D-aspartate (NMDA) glutamate-subtype receptor antagonist D-(-)-2-amino-5-phosphonopentanoate (APV; 0.1 mM). Glutamate exposure (0.1 mM and 1 mM) followed by 24 h of incubation caused the inhibition of succinate-cytochrome c reductase (20-25%) and cytochrome c oxidase (31%) activities in the surviving neurones, without affecting NADH-coenzyme-Q1 reductase activity. The rate of oxygen consumption was impaired in neurones exposed to 1 mM glutamate, either with glucose (by 26%) or succinate (by 39%) as substrates. These effects on the mitochondrial respiratory chain and neuronal respiration, together with the observed glutathione depletion (20%) by glutamate exposure were completely prevented by NAME or APV. Our results suggest that mitochondrial dysfunction and impairment of antioxidant status may account for glutamate-mediated neurotoxicity via a mechanism involving .NO biosynthesis.

Suppression of nitric oxide formation by tyrosine kinase inhibitors in murine N9 microglia

1. Microglial cells represent the first line of defence in the brain against infection and damage. However, under conditions of chronic inflammation and neurodegeneration, excessive activation of microglia can contribute to the neurodegenerative process by releasing a cornucopia of potentially cytotoxic substances including the cytotoxic free radical nitric oxide (NO). Although the cell signalling events implicated in NO formation in peripheral macrophages are well defined, events occurring in the phenotypically homologous cerebral microglial cell are not yet fully characterized. 2. In the present study, a cloned murine microglial cell line (N9), stimulated with combined lipopolysaccharide/interferon-gamma (LPS/IFN) incubation, was shown to produce a significant increase in NO formation, as measured by medium nitrite levels, during 8-72 h exposure. 3. LPS/IFN-stimulated NO production was partially inhibited with the nitric oxide synthase (NOS) competitive antagonists; N(omega)-nitro-L-arginine methyl ester and N(omega)-nitro-L-arginine. The ability of the selective inducible (iNOS) inhibitor, aminoguanidine, but not the selective 'neuronal-type' constitutive (cNOS) inhibitor 7-nitroindazole, to inhibit NO production suggested a primary role of iNOS in this response and was confirmed by immunolabelling of activated cells with a specific iNOS antibody. 4. A series of tyrosine kinase inhibitors, herbimycin A, genestein, tyrphostins, AG-126, AG-556 and the tyrosine phosphatase inhibitors, sodium orthovanadate and phenylarsine oxide, significantly attenuated LPS/IFN-mediated NO production. The serine/threonine kinase inhibitors, staursporine (protein kinase C), H-9 (cyclic GMP/cyclic AMP-dependent kinase) or serine/threonine phosphatase inhibitors, cyclosporin A (phosphatase 2B) and okadaic acid (phosphatase 1/2A), reduced NO formation by an apparent cytostatic mechanism, as determined by cellular reduction of 3-(4,5-dimethylthiazol-2-yi)-2,5-diphenyl-tetrazolium bromide (MTT). 5. The present results suggest that the co-ordinated activation of protein tyrosine kinases/phosphatases, and proximal signalling events implicating the interplay between serine-threonine kinases/phosphatases, is intricately linked with inflammatory mediated mechanisms of iNOS activation in microglial cells by regulating the activation of the transcription factor NFkappaB.

Microglia-derived nerve growth factor causes cell death in the developing retina

While nerve growth factor (NGF) is best known for its trophic functions, recent experiments indicate that it can also cause cell death during development by activating the neurotrophin receptor p75. We now identify microglial cells as the source of NGF as a killing agent in the developing eye. When the retina is separated from the surrounding tissue before colonization by microglial cells, no NGF can be detected, and cell death is dramatically reduced. It is restored by the addition of microglial cells, an effect that is blocked by NGF antibodies. NGF adsorbed at the surface of beads, but not soluble NGF, mimics the killing action of microglial cells. These results indicate an active role for macrophages in neuronal death.

Overexpression of superoxide dismutase and catalase in immortalized neural cells: toxic effects of hydrogen peroxide

Hydrogen peroxide (H2O2) is a known toxicant which causes its damage via the production of hydroxyl radicals. It has been reported to cause both necrotic and apoptotic cell death. The present study was undertaken to evaluate the mode of H2O2-induced cell death and to assess if overexpression of catalase could protect against its toxicity. H2O2 causes cell death of immortalized CSM 14.1 neural cells in a dose-dependent manner. H2O2-induced death was associated with DNA laddering as shown by agarose gel electrophoresis. Stable overexpression of catalase by transfection of a vector containing human cDNA into these cells markedly attenuated H2O2-induced toxic effects. Transfection of a vector containing a SOD cDNA afforded no protection. These results indicate that H2O2 can lead to the activation of endonuclease enzyme that breaks DNA into oligosomes. These cells which overexpress catalase or SOD will help to determine the specific role of H2O2 or O2- in the deleterious effects of a number of toxins.

Soluble macrophage factors trigger apoptosis in cultured hippocampal neurons

It is not clear whether macrophages which can phagocytose dead cells, may also contribute to death of potentially viable neurons when they enter brain lesion sites after insult. We have initially examined the effects of macrophage-conditioned medium on the integrity of hippocampal neurons in culture. We assessed qualitative and quantitative changes in neuronal status in terms of nuclear morphology, internucleosomal cleavage, cell membrane integrity and process density. Cell morphology with manual counts to quantitate findings showed that macrophage conditioned medium significantly increased the percentage of neurons with abnormal nuclei. Aurintricarboxylic acid attenuated this effect. Demonstration of laddering of DNA on agarose gels suggested an apoptosis-like event. A commercially available kit used to detect high concentrations of 3'-OH DNA ends showed marked increase in labelled cells. These combined findings confirmed that apoptosis was the main event triggered by conditioned medium. Although the number of cells with incompetent membranes also increased with conditioned medium application the majority of cells with apoptotic nuclei maintained membrane integrity. Conditioned medium also resulted in significant loss of cell processes. Conditioned medium from stimulated microglia showed a similar pattern of injury. The response of stressed neurons to conditioned medium was also tested. Exposure of cultures to mild hypoxia resulted in injury but did not significantly alter their subsequent vulnerability to macrophage-conditioned medium. Early experiments suggest that the documented changes in neuronal status are caused by relatively large and stable secreted macrophage proteins.

Experimental spinal cord injury: Wallerian degeneration in the dorsal column is followed by revascularization, glial proliferation, and nerve regeneration

The presence of adequate blood supply is a critical factor in recovery from traumatic injuries. We have examined whether the revascularization of the injured tissues is as crucial a precondition for wound healing in the spinal cord as in other organs. The development of the initial primary lesion (PL) after spinal crush injury in rats is followed by the formation of a unique tunnel-like dorsal column lesion (DCL) that extends rostrocaudally for many millimeters from the primary injury site. The DCL has been shown to result from Wallerian degeneration of the long spinal tracts in the dorsal column. In this study, we compared the processes of revascularization, wound healing, and nerve regeneration in the PL and the DCL by light microscopy after a crush injury of the cord. The spinal cord of 54 adults rats was crushed at T8 with jewelers forceps. The rats were allowed to survive from 3 h up to 8 weeks after spinal cord injury. The PL appeared immediately after injury and the DCL began to develop 6 h later. Infiltration of neutrophils, which is the first sign of the inflammatory responses to injury, began several hours later in the DCL than in the PL. Secondary vascular injury then occurred which resulted in hemorrhage around the DCL and rapid enlargement of the lesion during the remainder of the first week. Subsequent changes in the PL and DCL were entirely different. The PL underwent progressive enlargement and cavitation such that by 8 weeks, the lesion contained only very few cells, vessels, and axons scattered between huge fluid-filled cavities. The DCL, on the other hand, was maximal in size at 1 week and declined significantly in size and cavitation thereafter. By 8 weeks it was highly vascularized, contained abundant nerve fibers, and lacked any trace of cavitation. These findings amplify the current view that ischemia plays a critical role in spinal cord trauma by showing that revascularization precedes tissue repair and nerve regeneration in the dorsal columns. We conclude (a) that a well-vascularized lesion permits the ingrowth of glial and other cells which give rise to a supportive matrix for the nerve regeneration and (b) that procedures which induce revascularization or angiogenesis will ameliorate the cascade of progressive tissue necrosis.

Neurons promote macrophage proliferation by producing transforming growth factor-beta2

The infiltration of bone marrow-derived macrophages into the CNS contributes to growth and reactions of microglia during development or after brain injury. The proliferation of microglial cells is stimulated by colony-stimulating factor 1 (CSF-1), an astrocyte-produced growth factor that acts on mononuclear phagocytes. In the present study, we have shown, using an in vitro model system, that rodent neurons obtained from the developing cerebral cortex produce a soluble factor that strongly enhances the proliferation of macrophages cultured in the presence of CSF-1. Both macrophages isolated from the developing brain and those from the adult bone marrow were stimulated. Kinetic analyses of [3H]thymidine incorporation into macrophages indicated that their response to the neuron-derived factor involved a shortening of the cycle of proliferating cells. The effect of neurons on macrophages was blocked in the presence of antibodies neutralizing transforming growth factor-beta2 (TGF-beta2), whereas recombinant TGF-beta2 stimulated macrophage proliferation in the presence of CSF-1. Neuronal secretion of TGF-beta2 was confirmed by reverse transcription-PCR detection of TGF-beta2 transcripts and immunodetection of the protein within neurons and in their culture medium. In situ hybridization and immunohistochemical experiments showed neuronal expression of TGF-beta2 in sections of cerebral cortex obtained from 6-d-old rats, an age at which extensive developmental recruitment of macrophages occurs in this cerebral region. Altogether, our results provide direct evidence that neurons have the capacity to promote brain macrophage proliferation and demonstrate the role of TGF-beta2 in this neuronal function.