CNS glial scar tissue: a source of molecules which inhibit central neurite outgrowth

Relationship between sprouting axons, proteoglycans and glial cells following unilateral nigrostriatal axotomy in the adult rat

Proteoglycans may modulate axon growth in the intact and injured adult mammalian CNS. Here we investigate the distribution and time course of deposition of a range of proteoglycans between 4 and 14 days following unilateral axotomy of the nigrostriatal tract in anaesthetised adult rats. Immunolabelling using a variety of antibodies was used to examine the response of heparan sulphate proteoglycans, chondroitin sulphate proteoglycans and keratan sulphate proteoglycans. We observed that many proteoglycans became abundant between 1 and 2 weeks post-axotomy. Heparan sulphate proteoglycans were predominantly found within the lesion core (populated by blood vessels, amoeboid macrophages and meningeal fibroblasts) whereas chondroitin sulphate proteoglycans and keratan sulphate proteoglycans were predominantly found in the lesion surround (populated by reactive astrocytes, activated microglia and adult precursor cells). Immunolabelling indicated that cut dopaminergic nigral axons sprouted prolifically within the lesion core but rarely grew into the lesion surround. We conclude that sprouting of cut dopaminergic nigral axons may be supported by heparan sulphate proteoglycans but restricted by chondroitin sulphate proteoglycans and keratan sulphate proteoglycans.

Axons and glial interfaces: ultrastructural studies

At most vertebrate nerve transitional zones (TZs) there is a glial barrier which is pierced by axons passing between the CNS and PNS. Myelinated axons traverse this in individual tunnels. The same is true of larger non-myelinated axons. This holds widely among the vertebrates, for example, the large motor axons of the sea-lamprey Petromyzon (which also possess TZ specializations not found in mammals). Smaller non-myelinated axons traverse the TZ glial tunnels as fascicles and so the barriers are correspondingly less comprehensive for them. Accordingly, in nerves composed of non-myelinated axons, such as the vomeronasal or the olfactory, a TZ barrier stretching across the nerve is effectivelyabsent. The chordateAmphioxus differsfrom the vertebrates in lacking a TZ barrier throughout. Invertebrates also lack glial barriers at the TZs between ganglia and interconnecting nerve trunks. The glial barrier at the dorsal spinal root TZ (DRTZ) has considerable value for analysing protocols aimed at achieving CNS regeneration, because it provides a useful model of the gliotic reaction at sites of CNS injury. Also, it is especially amenable to morphometric analysis, and so enables objective quantification of different protocols. Being adjacent to the subarachnoid space, it is accessible for experimental intervention. The DRTZ was used to investigate the value of neurotrophin 3 (NT3) in promoting axon regeneration across the TZ barrier and into the CNS following dorsal root crush. It promoted extensive regeneration and vigorous non-myelinated axonal ensheathment. On average, around 40% of regenerating axons grew across the interface, compared with virtually none in its absence. These may have traversed the interface through loci occupied by axons prior to degeneration. Many regenerating axons became myelinated, both centrally and peripherally.

A method for purifying enteric glia from rat myenteric plexus

The enteric nervous system is a large and complex division of the peripheral nervous system. The glia associated with it share some characteristics with the olfactory-ensheathing glia, astrocytes and Schwann cells. To facilitate studies of rat enteric glia, we have developed a method for preparing them in large quantities with a high degree of homogeneity. The enteric glia were isolated from the small intestine of Wistar rats by enzymatic digestion with dispase. The cell isolate was added to a mitotically arrested layer of 3T3 cells. Subsequent separation of the enteric glia from the 3T3 cells was done enzymatically, with unavoidable loss of many enteric glia and potential contamination of enteric glia cultures with the 3T3 cells. Therefore, 3T3 cells were cultured in Nunc 0.2-microm tissue culture inserts that could be readily removed from the wells when no longer needed. There was no loss of the enteric glia. The cultures consisted entirely of GFAP-labeled cells, presumptive enteric glia. This method permits the culturing of large numbers of highly purified enteric glia without the use of expensive growth factors and complement-mediated cytolysis.

Cytokines regulate microglial adhesion to laminin and astrocyte extracellular matrix via protein kinase C-dependent activation of the alpha6beta1 integrin

Microglia are highly plastic cells that participate in inflammatory and injury responses within the CNS and that can migrate extensively after activation. Because astrocytes and their extracellular matrix (ECM) form a large part of the CNS parenchyma, we undertook to study the adhesive interactions between microglia and these substrates in vitro. In contrast to oligodendrocyte precursor cells, microglia formed only weak interactions with astrocytes and their ECM. On specific ECM substrates the microglia adhered strongly to fibronectin, vitronectin, and plastic but only weakly to laminin. Microglial adhesion to laminin was increased significantly by the proinflammatory cytokines TNF, IFN-alpha, and IFN-gamma but was decreased by TGF-beta1, with the TGF-beta1 effect being dominant over the other cytokines. Fluorescence-activated cell sorting (FACS) analysis and immunoprecipitation showed that microglia constitutively express the alpha6beta1 integrin, a well characterized laminin receptor, and that alpha6beta1 expression levels did not change after cytokine treatment. Function-blocking studies showed that microglial adhesion to laminin is mediated entirely by the alpha6beta1 integrin, strongly suggesting that the cytokine regulation of adhesion to laminin is mediated by changes in the activation state of alpha6beta1. Analysis of signaling pathways revealed that activation of alpha6beta1 is mediated by a PKC-dependent mechanism. In light of the evidence that laminin expression is upregulated after CNS injury, the findings suggest that cytokine regulation of microglial adhesion to laminin may play a fundamental role in determining the extent of microglial infiltration into and retention at the site of injury.

Induction of type IV collagen and other basement-membrane-associated proteins after spinal cord injury of the adult rat may participate in formation of the glial scar

We investigated the spatial and temporal expression of basement-membrane-forming and neurite-outgrowth-supporting matrix proteins after a unilateral dorsal root injury combined with a collagen I/laminin-1 graft and a stab wound lesion to the dorsal horn of the adult rat spinal cord. Ten days after injury, the gamma1 laminin was induced in the reactive glia. At this early stage, the glial cells failed to express type IV collagen and the alpha1 laminin. One month after injury, reactive astrocytes in the dorsal horn of the lesioned side expressed gamma1 laminin, type IV collagen, and the alpha1 laminin whereas astrocytes of the normal spinal cord or the uninjured contralateral dorsal horn were negative. Both astrocytes and neurons of the ipsilateral ventral horn were induced to express laminin-1 and gamma1 laminin. Astrocytes of the ipsilateral ventral horn also expressed type IV collagen. Simultaneously with the changes in expression of the extracellular matrix proteins, the expression pattern of basic fibroblast growth factor (FGF-2) was markedly altered after spinal cord injury. In normal and contralateral spinal cord, FGF-2 was expressed in nerve fibers, but its expression changed from neuronal into glial in the ipsilateral spinal cord within 1 month after injury. Four months after injury, expression of both type IV collagen and the alpha1 laminin had declined, but the astrocytes at the injury site continued expressing the gamma1 laminin. Cultured astrocytes were negative for type IV collagen, but several cytokines, including IL-1beta and TGFbeta1, induced expression of type IV collagen in the astrocytes. These factors also increased deposition of type IV collagen matrix in the glial cultures. These results indicate that type IV collagen and the alpha1 laminin are induced in reactive astrocytes after spinal cord injury in vivo. Induction of type IV collagen in astrocytes in vitro by cytokines indicates that blood-borne or local factors at the injury site may induce the spinal cord glial expression of type IV collagen in vivo. Simultaneous expression of laminin-1 and alpha1 laminin with type IV collagen is known to lead to production of basement membranes. This may hamper the neurite-outgrowth-promoting potential of the gamma1 laminin by initiating formation of the glial scar.

Ciliary neurotrophic factor activates spinal cord astrocytes, stimulating their production and release of fibroblast growth factor-2, to increase motor neuron survival

At focal CNS injury sites, several cytokines accumulate, including ciliary neurotrophic factor (CNTF) and interleukin-1beta (IL-1beta). Additionally, the CNTF alpha receptor is induced on astrocytes, establishing an autocrine/paracrine loop. How astrocyte function is altered as a result of CNTF stimulation remains incompletely characterized. Here, we demonstrate that direct injection of CNTF into the spinal cord increases GFAP expression and astroglial size and that primary cultures of spinal cord astrocytes treated with CNTF, IL-1beta, or leukemia inhibitory factor exhibit nuclear hypertrophy comparable to that observed in vivo. Using a coculture bioassay, we further demonstrate that CNTF treatment of astrocytes increases their ability to support ChAT(+) ventral spinal cord neurons (presumably motor neurons) more than twofold compared with untreated astrocytes. Also, the complexity of neurites was significantly increased in neurons cultured with CNTF-treated astrocytes compared with untreated astrocytes. RT-PCR analysis demonstrated that CNTF increased levels of FGF-2 and nerve growth factor (NGF) mRNA and that IL-1beta increased NGF and hepatocyte growth factor mRNA levels. Furthermore, both CNTF and IL-1beta stimulated the release of FGF-2 from cultured spinal cord astrocytes. These findings demonstrate that cytokine-activated astrocytes better support CNS neuron survival via the production of neurotrophic molecules. We also show that CNTF synergizes with FGF-2, but not epidermal growth factor, to promote DNA synthesis in spinal cord astrocyte cultures. The significance of these findings is discussed by presenting a new model depicting the sequential activation of astrocytes by cytokines and growth factors in the context of CNS injury and repair.

P2 receptor-types involved in astrogliosis in vivo

1. In the nucleus accumbens (NAc) of rats, the involvement of P2X and P2Y receptors in the generation of astrogliosis in vivo, was investigated by local application of their respective ligands. The agonists used had selectivities for P2X1,3 (alpha,beta-methylene adenosine 5'-triphosphate; alpha,beta-meATP), P2Y1,12 (adenosine 5'-O-(2-thiodiphosphate; ADP-beta-S) and P2Y2,4,6 receptors (uridine 5'-O-(3-thiotriphosphate; UTP-gamma-S). Pyridoxalphosphate-6-azophenyl-2,4-disulphonic acid (PPADS) was used as a non-selective antagonist. The astroglial reaction was studied by means of immunocytochemical double-labelling with antibodies to glial fibrillary acidic protein (GFAP) and 5-bromo-2'-deoxyuridine (BrdU). 2. The agonist-induced changes in comparison to the artificial cerebrospinal fluid (aCSF)-treated control side reveal a strong mitogenic potency of ADP-beta-S and alpha,beta-meATP, whereas UTP-gamma-S was ineffective. The P2 receptor antagonist PPADS decreased the injury-induced proliferation when given alone and in addition inhibited all agonist effects. 3. The observed morphogenic changes included hypertrophy of astrocytes, elongation of astrocytic processes and up-regulation of GFAP. A significant increase of both GFAP-immunoreactivity (IR) and GFA-protein content (by using Western blotting) was found after microinfusion of alpha,beta-meATP or ADP-beta-S. In contrast, UTP-gamma-S failed to increase the GFAP-IR. The morphogenic effects were also inhibited by pre-treatment with PPADS. 4. A double immunofluorescence approach with confocal laser scanning microscopy showed the localisation of P2X3 and P2Y1 receptors on the GFAP-labelled astrocytes. 5. In conclusion, the data suggest that P2Y (P2Y1 or P2Y12) receptor subtypes are involved in the generation of astrogliosis in the NAc of rats, with a possible minor contribution of P2X receptor subtypes.

Reactive glia support and guide axon growth in the rat thalamus during the first postnatal week. A sharply timed transition from permissive to non-permissive stage

The present study demonstrates a supportive and guiding effect of the reactive glia on the postlesional axon growth in vivo, and offers a model system to compare permissive and non-permissive forms of the glial reaction. After stab wounds in early postnatal (P2-P9) rats, the reactive glia and the nerve fibers were detected by the immunohistochemical staining of glial fibrillary acidic protein (GFAP) and neurofilament protein, respectively. In the thalamus of the animals lesioned at P5 or earlier, an extraordinary bundle of fibers immunoreactive to neurofilament protein was found, corresponding to the lesion track marked by reactive glia. This bundle persisted up to 2 months, as shown by electron microscopy. When the animals were lesioned at P7 or later, the lesion track was immunonegative to neurofilament protein. Following P6 lesions, an intermediate situation was found, the strip of immunoreactive neurofilament protein was missing, or short and weak. GFAP immunostaining demonstrated a typical reactive glia in every case. As a result of the same operation, reactive glia plus a deficiency of neurofilament protein immunostaining was found in every animal in the cortex and the corpus callosum, independently from the age at lesion. The results demonstrate that the permissive nature of the glial reaction depends on the lesioned area as well, and changes to a non-permissive effect in a short time interval.

Neutralizing antibodies against neurite growth inhibitor NI-35/250 do not promote regeneration of sensory axons in the adult rat spinal cord

Neutralization of the myelin-associated neurite growth inhibitors NI-35 and NI-250 by IN-1 antibodies can promote axonal regeneration of several types of central nervous neurons. Here, we investigated in adult rats whether IN-1 can promote regeneration of ascending sensory axons across a peripheral nerve bridge back into the spinal cord. IN-1 was administered by hybridoma cells injected in the cerebral cortex or thoracic cord, its presence confirmed in tissue sections and cerebrospinal fluid, and its effectiveness demonstrated in co-cultures of oligodendrocytes and sensory neurons. With a two week infusion of control vehicle into the dorsal spinal cord 3 mm rostral to the nerve graft, only 3+/-2% of the anterogradely labeled sensory fibers present at the rostral end of the nerve graft had grown up to 0.5 mm, but not farther into the spinal cord. A similar limited extent of regeneration was seen with IN-1 or with infusion of Dantrolene, an inhibitor of NI-35/250 activity in vitro. With infusion of nerve growth factor rostral to the nerve graft, 40% of the fibers at the rostral end of the graft were found at 0.5 mm, 34% at 1 mm, 24% at 2 mm and 14% at 3 mm (the infusion site) into the spinal cord. Treatment with IN-l antibodies did not enhance the growth-promoting effects of nerve growth factor. We suggest that the neurite growth inhibitors NI-35 or NI-250 do not play a major inhibitory role in the regeneration of the ascending sensory fibers across a nerve bridge and back into the spinal cord of the adult rat.

Response of abducens internuclear neurons to axotomy in the adult cat

The highly specific projection of abducens internuclear neurons on the medial rectus motoneurons of the oculomotor nucleus constitutes an optimal model for investigating the effects of axotomy in the central nervous system. We have analyzed the morphological changes induced by this lesion on both the cell bodies and the transected axons of abducens internuclear neurons in the adult cat. Axotomy was performed by the transection of the medial longitudinal fascicle. Cell counts of Nissl-stained material and calretinin-immunostained abducens internuclear neurons revealed no cell death by 3 months postaxotomy. Ultrastructural examination of these cells at 6, 14, 24, and 90 days postaxotomy showed normal cytological features. However, the surface membrane of axotomized neurons appeared contacted by very few synaptic boutons compared to controls. This change was quantified by measuring the percentage of synaptic coverage of the cell bodies and the linear density of boutons. Both parameters decreased significantly after axotomy, with the lowest values at 90 days postlesion ( approximately 70% reduction). We also explored axonal regrowth and the possibility of reinnervation of a new target by means of anterograde labeling with biocytin. At all time intervals analyzed, labeled axons were observed to be interrupted at the caudal limit of the lesion; in no case did they cross the scar tissue to reach the distal part of the tract. Nonetheless, a conspicuous axonal sprouting was present at the caudal aspect of the lesion site. Structures suggestive of axonal growth were found, such as large terminal clubs, from which short filopodium-like branches frequently emerged. Similar findings were obtained after parvalbumin and calretinin immunostaining. At the electron microscopy level, biocytin-labeled boutons originating from the sprouts appeared surrounded by either extracellular space, which was extremely dilated at the lesion site, or by glial processes. The great majority of labeled boutons examined were, thus, devoid of neuronal contact, indicating absence of reinnervation of a new target. Altogether, these data indicate that abducens internuclear neurons survive axotomy in the adult cat and show some form of axonal regrowth, even in the absence of target connection.

Nervous system proteoglycans as modulators of neurite outgrowth

The proteoglycans are multifunctional macromolecules composed of a core polypeptide and a variable number of glycosaminoglycan chains. The structural diversity and complexities of proteoglycan expression in the developing and adult Nervous System underlies the variety of biological functions that these molecules fulfill. Thus, in the Nervous System, proteoglycans regulate the structural organisation of the extracellular matrix, modulate growth factor activities and cellular adhesive and motility events, such as cell migration and axon outgrowth. This review summarises the evidences indicating that proteoglycans have an important role as modulators of neurite outgrowth and neuronal polarity. Special emphasis will be placed on those studies that have shown that proteoglycans of certain subtypes inhibit neurite extension either during the development and/or the regeneration of the vertebrate Central Nervous System.

Chondroitinase ABC promotes axonal regeneration of Clarke's neurons after spinal cord injury

We examined whether enzymatic digestion of chondroitin sulfate (CS) promoted the axonal regeneration of neurons in Clarke's nucleus (CN) into a peripheral nerve (PN) graft following injury of the spinal cord. After hemisection at T11, a segment of PN graft was implanted at the lesion site. Either vehicle, brain-derived neurotrophic factor (BDNF) or chondroitinase ABC was applied at the implantation site. The postoperative survival period was 4 weeks. Treatment with vehicle or BDNF did not promote the axonal regeneration of CN neurons into the PN graft. Application of 2.5 unit/ml chondroitinase ABC resulted in a significant increase (12.8%) in the number of regenerated CN neurons. Double labeling with Fluoro-Gold and NADPH-diaphorase histochemistry showed that the regenerated CN neurons did not express nitric oxide synthase (NOS). Our results suggest that CS is inhibitory to the regeneration of CN neurons following injury of the spinal cord.

Limits to the capacity of transplants of olfactory glia to promote axonal regrowth in the CNS

Olfactory bulb ensheathing cell (OBEC) transplants promoted axonal regeneration in the spinal cord dorsal root entry zone and in the corticospinal tract. However, OBECs failed to promote abducens internuclear neuron axon regeneration when transplanted at the site of nerve fibre transection. In experiments performed in both cats and rats, OBECs survived for up to 2 months, lining themselves up along the portion of the regrowing axons proximal to the interneuron cell body. However, OBECs migrated preferentially towards abducens somata, in the direction opposite to the oculomotor nucleus target. OBECs seem to promote nerve fibre regeneration only where preferred direction of glial migration coincides with the direction of axonal growth towards its target.

Functional recovery of paraplegic rats and motor axon regeneration in their spinal cords by olfactory ensheathing glia

Axonal regeneration in the lesioned mammalian central nervous system is abortive, and this causes permanent disabilities in individuals with spinal cord injuries. In adult rats, olfactory ensheathing glia (OEG) transplants successfully led to functional and structural recovery after complete spinal cord transection. From 3 to 7 months post surgery, all OEG-transplanted animals recovered locomotor functions and sensorimotor reflexes. They presented voluntary hindlimb movements, they supported their body weight, and their hindlimbs responded to light skin contact and proprioceptive stimuli. In addition, relevant motor axons (corticospinal, raphespinal, and coeruleospinal) regenerated for long distances within caudal cord stumps. Therefore, OEG transplantation provides a useful repair strategy in adult mammals with traumatic spinal cord injuries. Our results with these cells could lead to new therapies for the treatment of spinal cord lesions in humans.

Neurite outgrowth inhibitors in gliotic tissue

Gliotic tissue is the major obstacle to axon regeneration after CNS injury. We designed tissue culture assays to search for molecules responsible for neurite outgrowth inhibition in gliotic tissue. All the inhibitory activity in injured brain tissue was located in a plasma membrane heparan-sulphate and condroitin-sulphate type-proteoglycan of apparent molecular weight 200 kDalton. The proteoglycan core protein (apparent MW 48,000 kD) was biologically inactive, whereas the glycosamine-glycan (GAG) chains accounted for the inhibitory activity. Because of its cell location and mode of induction, the inhibitor was called injured membrane proteoglycan, IMP. IMP prevented neurite outgrowth initiation when attached to the culture substrate and caused growth cone collapse when added in solution to neurons with already growing neurites. We concluded that IMP was responsible for preventing injured CNS fibre regeneration. Double-staining immunohistochemistry of normal and gliotic tissue with anti-IMP monoclonal antibodies together with glial and neuronal markers, permitted the unequivocal definition of inhibitor presenting cells by confocal microscopy. IMP-immunostaining in normal CNS was observed exclusively on neurons. However, after a lesion, immunostaining occurred primarily on intensely GFAP-positive reactive astrocytes, but not on OX-42 positive microglia. The availability of antibodies permitted rapid affinity-purification of the neurite inhibitor and comparison with similar molecules possibly expressed during development. IMP itself or a highly related form, was expressed in embryonic brain, reaching maximal expression around postnatal day 3 and decreasing strongly in normal adult tissue. Perinatal rat brain proteoglycans inhibited neurite outgrowth similarly, though not identically, to IMP. Our data suggest that perinatal membrane and injured membrane proteoglycans may differ in GAG composition. IMP-like immunoreactivity was also found in developing brain, predominantly in neurons in normal brain, associating after a lesion with reactive astrocytes. Thes results suggest that injury evokes re-expression of IMP previously expressed during CNS development. One of the monoclonal antibodies to IMP blocked inhibitory activity, restoring neurite outgrowth in vitro. We are currently preparing Fab fragments to test the possibility that the antibody may block inhibition of central sprout growth in vivo. The combined use of blocking antibody fragments to neurite outgrowth inhibitors and transplants of growth-promoting glia, may help in the repair brain and spinal cord lesions.

White matter of the CNS supports or inhibits neurite outgrowth in vitro depending on geometry

Axonal regeneration is normally limited within myelinated fiber tracts in the CNS of higher vertebrates. Numerous studies suggest that CNS myelin contains inhibitors that may contribute to abortive axonal growth. In contrast to the evidence of myelin-associated neurite inhibitors, embryonic neurons transplanted into the CNS can regenerate extensively within myelinated tracts in vivo. It has been speculated that embryonic neurons do not yet express the appropriate receptors for myelin-associated inhibitors. Recently, however, extensive regeneration from transplanted adult neurons has also been reported within myelinated tracts of the CNS, casting doubt on the role myelin-associated inhibitors play in abortive regeneration. The present study reexamined the potential of white matter to support neurite growth in vitro. By the use of Neurobasal medium, neurons were cultured onto unfixed cryostat sections of mature rat CNS tissue. As documented previously, robust neuronal attachment and neurite outgrowth occurred on gray matter but these neurites were sharply inhibited by white matter. In addition, however, increased rates of neuronal attachment directly to white matter occurred with neurite outgrowth comparable in length with that on gray matter but limited to directions parallel to the fiber tract. Frequently, the same section of white matter was found to inhibit neurite outgrowth from neurons on gray matter while supporting parallel neurite outgrowth from neurons on white matter. These results suggest that whether white matter supports or inhibits axonal growth depends on the geometric relationship between the axon and the fiber tract; more specifically, white matter supports parallel growth but inhibits nonparallel growth.

Combined fetal neural transplantation and nerve growth factor infusion: effects on neurological outcome following fluid-percussion brain injury in the rat

This study was designed to evaluate the histological and behavioral impact of fetal neural transplantation with and without neurotrophin infusion in rats subjected to traumatic brain injury using a clinically relevant model of lateral fluid-percussion brain injury. Adult male Sprague-Dawley rats received lateral fluid-percussion brain injury of moderate severity (2.1-2.3 atm). Twenty-four hours after injury, minced fetal cortical grafts (E16) were stereotactically transplanted into the site of injury cavity formation (in 32 rats). Ten control animals received injections of saline. A third group of 29 animals that received transplants also underwent placement of a miniosmotic pump (immediately after transplantation) to continuously infuse nerve growth factor (NGF) directly into the region of graft placement for the duration of the experiment. A fourth group of eight animals underwent transplantation of fetal cortical cells that had been dissociated and placed in suspension. Animals were evaluated at 72 hours, 1 week, and 2 weeks after injury for cognitive function (using the Morris water maze), posttraumatic motor dysfunction, and transplant survival and morphology (using Nissl and modified Palmgren's silver staining techniques). Robust survival of whole-tissue transplants was seen in 65.6% of animals and was not increased in animals receiving NGF infusion. Animals receiving transplants of cell suspension had no surviving grafts. Brain-injured animals receiving transplants showed significant cognitive improvements compared with controls at the 2-week evaluation. Significantly improved memory scores were seen at all evaluation times in animals receiving both NGF and transplants compared with injured controls and compared with animals receiving transplants alone at the 72-hour and 1-week evaluations. Neurological motor function scores were significantly improved in animals receiving transplants alone and those receiving transplants with NGF infusion. Histological evaluation demonstrated differentiation of grafted cells, decreased glial scarring around transplants when compared with control animals, and the presence of neuronal fibers bridging the interface between graft and host. This study demonstrates that fetal cortical cells transplanted into the injured cortex of the adult rat can improve both posttraumatic cognitive and motor function and interact with the injured host brain.

Modulation of apolipoprotein D and apolipoprotein E expression in rat hippocampus after entorhinal cortex lesion

Apolipoprotein (apo) D is a member of the lipocalin family of proteins. Although its physiological function is unknown, apoD is thought to transport one or more small hydrophobic ligands. A second apolipoprotein, apoE is known to play an important role in lipid transport, and apoE genetic polymorphism has been shown to be associated with susceptibility to Alzheimer's disease. Both apoD and apoE are expressed in the central nervous system (CNS) and both proteins accumulate at sites of peripheral nerve injury due to increased local synthesis. The two proteins may have overlapping or complementary functions within nervous tissue. In order to define the role of apoD within the CNS, we have studied the regional distribution of apoD and apoE mRNA and protein within the normal rat brain and the changes in apoD and apoE expression in the hippocampus of rats after entorhinal cortex lesion (EC lesion). Within the brains of normal rats, apoD expression in the hippocampus was as high as 180-fold that of the liver. ApoD mRNA levels in other areas of the rat brain ranged from 40 to 120 times the hepatic levels. The distribution of apoE gene expression within the brain was similar to that of apoD, but was much lower than hepatic apoE expression. When rats were subjected to EC lesion, the apoD message increased by 54% at 4 days post lesion (DPL) in the ipsilateral region of hippocampus while apoE mRNA levels (ipsilateral and contralateral) decreased by 43%. At 6 to 8 DPL apoD mRNA in the ipsilateral hippocampus remained elevated (42% above controls) whereas the apoE mRNA levels increased to about 15% above those of controls. At 14 and 31 DPL, both apoD and apoE expression was similar to controls. The increase in immunoreactive apoD in hippocampal extracts was more dramatic. At 1 DPL, immunoreactive apoD levels were already 16-fold higher than those in extracts of non-lesioned animals and, at 31 DPL, levels were still 8-fold higher than those of control animals. Finally, we have demonstrated that the levels of apoD in the brains of apoE-deficient mice are 50-fold those of wildtype control mice. ApoD clearly has an important function within the CNS in both normal and pathological situations.

Up-regulation of Eph tyrosine kinase receptors after excitotoxic injury in adult hippocampus

The molecular mechanisms underlying the response to injury in the central nervous system are incompletely understood. Many cell activation systems may be involved. Tyrosine kinase receptors and their ligands play key roles in cell activation throughout life. The Eph family of tyrosine kinase receptors/ ligands are developmentally regulated and have been implicated in neural pathfinding. However, nothing is known about their role in the adult brain. We have used a model of central nervous system lesion in the rat, in which intraventricular injection of kainate was performed. This produced neuronal death in the CA3-CA4 fields and glial activation in the hippocampus. Highly degenerate primers, corresponding to the catalytic domain of the tyrosine kinase family, were used for reverse transcription-polymerase chain reaction of pooled RNA extracted from injured hippocampi. The amplified products were cloned and 100 clones (arbitrarily named TK1-TK100) were examined and inserts sequenced. We obtained four clones containing inserts which belong to the Eph receptor family. Two of these inserts (TK17 and TK63) were EphA4 and the other were EphB2 (TK25) and EphA5 (TK23). We performed in situ hybridization, and we found our clones to be present in all fields of the hippocampus, their expression being mainly neuronal. Three days after lesion, prominent expression appeared in CA1 as compared to the same field in the non-treated contralateral hippocampus. We performed northern blot analysis for quantification, and found that, three days after injury, the values decreased to 33 +/- 4%, 33 +/- 1% and 46 +/- 1% of control values for TK63 (EphA4), TK25 (EphB2) and TK23 (EphA5), respectively. Neuronal death in CA3-CA4 might account for this fact. Later, five days post-injury, the expression increased to 63 +/- 3%, 71 +/- 1% and 111 +/- 5% of control values, respectively. This increase was due to an up-regulation of these genes in the hippocampal neurons that survive after the injury, as indicated by in situ hybridization.

Dehydroepiandrosterone, pregnenolone and sex steroids down-regulate reactive astroglia in the male rat brain after a penetrating brain injury

Astrocytes are a target for steroid hormones and for steroids produced by the nervous system (neurosteroids). The effect of gonadal hormones and several neurosteroids in the formation of gliotic tissue has been assessed in adult male rats after a penetrating wound of the cerebral cortex and the hippocampal formation. The hormones testosterone, 17beta-estradiol and progesterone and the neurosteroids dehydroepiandrosterone, pregnenolone and pregnenolone sulfate resulted in a significant decrease in the accumulation of astrocytes in the proximity of the wound and in a decreased bromodeoxyuridine incorporation in reactive astrocytes. Of all steroids tested, dehydroepiandrosterone was the most potent inhibitor of gliotic tissue formation. These findings suggest that neurosteroids and sex steroids may affect brain repair by down-regulating gliotic tissue.

Cerebral astrocyte response to micromachined silicon implants

The treatment of neurologic disorders and the restoration of lost function due to trauma by neuroprosthetic devices has been pursued for over 20 years. The methodology for fabricating miniature devices with sophisticated electronic functions to interface with nervous system tissue is available, having been well established by the integrated circuit industry. Unfortunately, the effectiveness of these devices is severely limited by the tissue reaction to the insertion and continuous presence of the implant, a foreign object. This study was designed to document the response of reactive astrocytes in the hope that this information will be useful in specifying new fabrication technologies and devices capable of prolonged functioning in the brain. Model probes fabricated from single crystal silicon wafers were implanted into the cerebral cortices of rats. The probes had a 1 x 1-mm tab, for handling, and a 2-mm-long shaft with a trapezoidal cross-section (200-microm base, 60microm width at the top, and 130 microm height). The tissue response was studied by light and scanning electron microscopy at postinsertion times ranging from 2 to 12 weeks. A continuous sheath of cells was found to surround the insertion site in all tissue studied and was well developed but loosely organized at 2 weeks. By 6 and 12 weeks, the sheath was highly compacted and continuous, isolating the probe from the brain. At 2 and 4 weeks, the sheath was disrupted when the probe was removed from the fixed tissue, indicating that cells attached more strongly to the surface of the probe than to the nearby tissue. The later times showed much less disruption. Scanning electron microscopy of the probes showed adherent cells or cell fragments at all time points. Thus, as the sheath became compact, the cells on the probe and the cells in the sheath had decreased adhesion to each other. Immunocytochemistry demonstrated that the sheath was labeled with antibodies to glial fibrillary acidic protein (GFAP), an indicator for reactive gliosis. The tissue surrounding the insertion site showed an increased number of GFAP-positive cells which tended to return to control levels as a function of time after probe insertion. It was concluded that reactive gliosis is an important part of the process forming the cellular sheath. Further, the continuous presence of the probe appears to result in a sustained response that produces and maintains a compact sheath, at least partially composed of reactive glia, which isolates the probe from the brain.

Ensheathing glia transplants promote dorsal root regeneration and spinal reflex restitution after multiple lumbar rhizotomy

Previously, we have shown that transplants of olfactory bulb ensheathing cells promoted regeneration of transected dorsal roots into the spinal cord. In this study, we assessed the ability of regenerating axons to make functional connections in the cord. Dorsal roots L3 to L6 were sectioned close to their entrance into the spinal cord and reapposed after injecting a suspension of ensheathing cells into each dorsal root entry zone (Group G). Afferent regeneration into the cord and recovery of spinal reflexes were compared with animals that received no injection (Group S) or culture medium without cells (Group C). Electrophysiological tests, to measure nerve conduction and spinal reflexes (H response and withdrawal reflex) evoked by stimulation of afferents of the sciatic nerve, were performed. At 14 days after surgery, H response was found in only 1 of 7 rats of Group G, and withdrawal reflexes were absent from all animals. At 60 days, the H response reappeared in 7 of 10 rats of Group G, and 1 of 5 of each of Groups C and S. The withdrawal reflex recovered in 4 of 10 rats of Group G, but in none of Groups C and S. Immunohistochemical labeling for calcitonin gene-related peptide (CGRP) in rats of Group G showed immunoreactive fibers entering the dorsal horn from sectioned roots, although at lower density than in the contralateral side. In conclusion, transplanted ensheathing cells promoted central regeneration and functional reconnection of regenerating sensory afferents.

The transitional zone and CNS regeneration

Most nerves are attached to the neuraxis by rootlets. The CNS-PNS transitional zone (TZ) is that length of rootlet containing both central and peripheral nervous tissue. The 2 tissues are separated by a very irregular but clearly defined interface, consisting of the surface of the astrocytic tissue comprising the central component of the TZ. Central to this, myelin sheaths are formed by oligodendrocytes and the supporting tissue is astrocytic. Peripheral to it, sheaths are formed by Schwann cells which are enveloped in endoneurium. The features of transitional nodes are a composite of those of central and peripheral type. The interface is penetrated only by axons. It is absent at first. It is formed by growth of processes into the axon bundle from glial cell bodies around its perimeter. These form a barrier across the bundle which fully segregates prospectively myelinated axons. Rat spinal dorsal root TZs have been used extensively to study CNS axon regeneration. The CNS part of the TZ responds to primary afferent axon degeneration and to regenerating axons in ways which constitute a satisfactory model of the gliotic tissue response which occurs in CNS lesions. It undergoes gliosis and the gliotic TZ tissue expands distally along the root. In mature animals axons can regenerate satisfactorily through the endoneurial tubes of the root but cease growth on reaching the gliotic tissue. The general objective of experimental studies is to achieve axon regeneration from the PNS through this outgrowth and into the dorsal spinal cord. Since immature tissue has a greater capacity for regeneration than that of the adult, one approach includes the transplantation of embryonic or fetal dorsal root ganglia into the locus of an extirpated adult ganglion. Axons grow centrally from the transplanted ganglion cells and some enter the cord. Other approaches include alteration of the TZ environment to facilitate axon regeneration, for example, by the application of tropic, trophic, or other molecular factors, and also by transplantation of cultured olfactory ensheathing cells (OECs) into the TZ region. OECs, by association with growing axons, facilitate their extensive regeneration into the cord. Unusually, ventral motoneuron axons may undergo some degree of unaided CNS regeneration. When interrupted in the spinal cord white matter, some grow out to the ventral rootlet TZ and thence distally in the PNS. The DRTZ is especially useful for quantitative studies on regeneration. Since the tissue is anisometric, individual parameters such as axon numbers, axon size and glial ensheathment can be readily measured and compared in the CNS and PNS environments, thereby yielding indices of regeneration across the interface for different sets of experimental conditions.

Evidence for a role of the chemorepellent semaphorin III and its receptor neuropilin-1 in the regeneration of primary olfactory axons

To explore a role for chemorepulsive axon guidance mechanisms in the regeneration of primary olfactory axons, we examined the expression of the chemorepellent semaphorin III (sema III), its receptor neuropilin-1, and collapsin response mediator protein-2 (CRMP-2) during regeneration of the olfactory system. In the intact olfactory system, neuropilin-1 and CRMP-2 mRNA expression define a distinct population of olfactory receptor neurons, corresponding to immature (B-50/GAP-43-positive) and a subset of mature (olfactory marker protein-positive) neurons located in the lower half of the olfactory epithelium. Sema III mRNA is expressed in pial sheet cells and in second-order olfactory neurons that are the target cells of neuropilin-1-positive primary olfactory axons. These data suggest that in the intact olfactory bulb sema III creates a molecular barrier, which helps restrict ingrowing olfactory axons to the nerve and glomerular layers of the bulb. Both axotomy of the primary olfactory nerve and bulbectomy induce the formation of new olfactory receptor neurons expressing neuropilin-1 and CRMP-2 mRNA. After axotomy, sema III mRNA is transiently induced in cells at the site of the lesion. These cells align regenerating bundles of olfactory axons. In contrast to the transient appearance of sema III-positive cells at the lesion site after axotomy, sema III-positive cells increase progressively after bulbectomy, apparently preventing regenerating neuropilin-1-positive nerve bundles from growing deeper into the lesion area. The presence of sema III in scar tissue and the concomitant expression of its receptor neuropilin-1 on regenerating olfactory axons suggests that semaphorin-mediated chemorepulsive signal transduction may contribute to the regenerative failure of these axons after bulbectomy.

A neurite outgrowth-inhibitory proteoglycan expressed during development is similar to that isolated from adult brain after isomorphic injury

The expression of proteoglycans (PGs) in the mammalian central nervous system (CNS) appears to be strictly regulated both during development and after damage to the mammalian CNS. Recently, we have isolated from membranes of injured adult brain a neurite outgrowth-inhibitory proteoglycan (IMP), the activity of which could be specifically counteracted by a monoclonal antibody (mAB) against the PG. We described in this report the characterization of perinatal membrane proteoglycan (PMP), a heparan-sulfate/chondroitin-sulfate-containing PG expressed during brain development. Its maximal expression was observed around postnatal day 3, decreasing strongly in normal adult tissue. This PG was purified and characterized using mABs generated against IMP. The comparison of PMP and IMP properties indicates that the two PGs are highly related and share expression patterns, biochemical characteristics, and the ability to inhibit neurite initiation in culture. However, IMP and PMP displayed a distinct effect on neurite elongation, which may be explained by their differences in glycosilation pattern. The data presented in this report support the idea that proteoglycans expressed during CNS development are re-expressed following injury.

Long-distance axonal regeneration in the transected adult rat spinal cord is promoted by olfactory ensheathing glia transplants

The lack of axonal regeneration in the injured adult mammalian spinal cord leads to permanent functional impairment. To induce axonal regeneration in the transected adult rat spinal cord, we have used the axonal growth-promoting properties of adult olfactory bulb ensheathing glia (EG). Schwann cell (SC)-filled guidance channels were grafted to bridge both cord stumps, and suspensions of pure (98%) Hoechst-labeled EG were stereotaxically injected into the midline of both stumps, 1 mm from the edges of the channel. In EG-transplanted animals, numerous neurofilament-, GAP-43-, anti-calcitonin gene-related peptide (CGRP)-, and serotonin-immunoreactive fibers traversed the glial scars formed at both cord-graft interfaces. Supraspinal serotonergic axons crossed the transection gap through connective tissue bridges formed on the exterior of the channels, avoiding the channel interior. Strikingly, after crossing the distal glial scar, these fibers elongated in white and periaqueductal gray matter, reaching the farthest distance analyzed (1.5 cm). Tracer-labeled axons present in SC grafts were found to extend across the distal interface and up to 800 microm beyond in the distal cord. Long-distance regeneration (at least 2.5 cm) of injured ascending propriospinal axons was observed in the rostral spinal cord. Transplanted EG migrated longitudinally and laterally from the injection sites, reaching the farthest distance analyzed (1.5 cm). They moved through white matter tracts, gray matter, and glial scars, overcoming the inhibitory nature of the CNS environment, and invaded SC and connective tissue bridges and the dorsal and ventral roots adjacent to the transection site. Transplanted EG and regenerating axons were found in the same locations. Because EG seem to provide injured spinal axons with appropriate factors for long-distance elongation, these cells offer new possibilities for treatment of CNS conditions that require axonal regeneration.

Olfactory ensheathing glia: properties and function

The failure of regenerating axons to grow within the adult mammalian central nervous system (CNS) does not apply to the olfactory bulb (OB). In this structure, normal and transected olfactory axons are able to enter, regenerate, and reestablish lost synaptic contacts with their targets, throughout the lifetime of the organism. A remarkable difference between an axonal growth-permissive structure such as the OB and the remaining CNS resides in the presence of ensheathing glia in the former. These cells exhibit phenotypic and functional properties known to be involved in the process of axonal elongation that may explain the permissibility of the OB to axonal growth. In addition, transplants of ensheathing glia were successfully used to promote axonal regeneration within the injured adult CNS. The axonal growth-promoting properties of ensheathing glia make the study of this cell type interesting to provide an insight into the mechanisms underlying the process of axonal regeneration. Therefore, in this article we review the developmental, morphologic, immunocytochemical, and functional properties presented by this unique glial cell type, and correlate them with the axonal growth-promoting ability of ensheathing glia. In addition, we provide some evidence of the potentiality that ensheathing glia might have as a promoter of axonal regeneration within the injured nervous system.

Transmural compression-induced proliferation and DNA synthesis through activation of a tyrosine kinase pathway in rat astrocytoma RCR-1 cells

Gliosis results from abnormal proliferation of glial cells and often occurs in response to brain or spinal cord injury. There are many factors that trigger gliosis associated with such injuries, including ischemia, humoral factors produced by the injured tissue, and possibly mechanical compression itself. In the present study, the effects of mechanical compression on cell proliferation and DNA synthesis were examined in vitro with the rat astrocyte cell line RCR-1. Pressure was applied to cells by instilling compressed helium into sealed plates or flasks in which the partial pressure of oxygen were maintained constant. Compression resulted in time- and intensity-dependent increases in cell number and [3H]thymidine incorporation, with maximum effects apparent at 10 min and 120 mmHg. Compression-induced cell proliferation and DNA synthesis were not inhibited by gadolinium (Gd3+), a blocker of stretch-activated ion channels, or by inhibitors of protein kinase A, protein kinase C, or Ca2+/calmodulin-dependent protein kinases. However, the tyrosine kinase inhibitor genistein inhibited these effects of compression in a concentration-dependent manner. Conditioned medium from compressed cells also induced cell proliferation and DNA synthesis at atmospheric pressure in a genistein-sensitive manner. These results suggest that transmural compression triggers the release of a factor (or factors) that induces cell proliferation and DNA synthesis through a tyrosine kinase pathway in RCR-1 cells.

c-Jun expression in adult rat dorsal root ganglion neurons: differential response after central or peripheral axotomy

The response of the mature central nervous system (CNS) to injury differs significantly from the response of the peripheral nervous system (PNS). Axotomized PNS neurons generally regenerate following injury, while CNS neurons do not. The mechanisms that are responsible for these differences are not completely known, but both intrinsic neuronal and extrinsic environmental influences are likely to contribute to regenerative success or failure. One intrinsic factor that may contribute to successful axonal regeneration is the induction of specific genes in the injured neurons. In the present study, we have evaluated the hypothesis that expression of the immediate early gene c-jun is involved in a successful regenerative response. We have compared c-Jun expression in dorsal root ganglion (DRG) neurons following central or peripheral axotomy. We prepared animals that received either a sciatic nerve (peripheral) lesion or a dorsal rhizotomy in combination with spinal cord hemisection (central lesion). In a third group of animals, several dorsal roots were placed into the hemisection site along with a fetal spinal cord transplant. This intervention has been demonstrated to promote regrowth of severed axons and provides a model to examine DRG neurons during regenerative growth after central lesion. Our results indicated that c-Jun was upregulated substantially in DRG neurons following a peripheral axotomy, but following a central axotomy, only 18% of the neurons expressed c-Jun. Following dorsal rhizotomy and transplantation, however, c-Jun expression was upregulated dramatically; under those experimental conditions, 63% of the DRG neurons were c-Jun-positive. These data indicate that c-Jun expression may be related to successful regenerative growth following both PNS and CNS lesions.

Regulation by interleukin-1beta of formation of a line of delimiting astrocytes following prenatal trauma to the brain of the mouse

The regulation of perinatal glia limitans (GL) reformation by interleukin-1beta (IL-1beta) following prenatal neural trauma in the mouse was studied in lesioned fetal mice by immunocytochemistry and computer-assisted image analysis for presence and distribution of astrocytes and IL-1beta immunoreactivity (ir). Astrocytes stained with anti-glial fibrillary acidic protein (GFAP) were observed as a line of delimiting astrocytes (LDA) near the lesion edge on Postnatal Day 0 (P0, 2 days postlesion). At P6, a new and complete GL composed of GFAP-positive astrocytes was continuous with that of adjacent undamaged tissue. The new GL was located in the same area at P6 as was the LDA at P0, suggesting that the LDA is the precursor structure to a reformed GL. Astrocytes comprising the new GL were positive for anti-IL-1beta. The IL-1 receptor antagonist (IL-1ra), administered acutely into the lesion, produced a significantly decreased optical density of IL-1beta-ir at the LDA at P0 compared to animals that received injections of vehicle, human recombinant IL-1beta, or a combination injection of IL-1ra + IL-1beta. Furthermore, although GFAP-stained cells appeared at the lesion site, an organized LDA was not visible at P0 in IL-1ra-treated animals. Vehicle-, IL-1beta-, and combination-injected animals showed a robust LDA at the lesion site at P0. These data suggest that upregulation of IL-1beta in astrocytes and interaction of IL-1beta with the neural IL-1 receptor are important for reconstruction of the GL following prenatal lesion in the murine brain.

Neurite outgrowth inhibitor of gliotic brain tissue. Mode of action and cellular localization, studied with specific monoclonal antibodies

Membranes from injured adult rat brain express a heparan/chondroitin sulphate proteoglycan that inhibits neurite outgrowth in vitro. We have developed monoclonal antibodies (Mabs) against this proteoglycan, two of which were characterized and used for the study of the inhibitor mode of action and localization in normal and injured adult brain. The antibodies recognized a molecule of apparent molecular weight 200 kDa in Western blots of injured brain membranes. One of the Mabs blocked both the inhibition of neurite outgrowth and the growth cone collapse activity, associated with the proteoglycan. In adult brain, inhibitor immunoreactivity was found predominantly in neurons but, after a lesion, it was associated mainly with reactive glial cells. The localization of neurite outgrowth inhibitors in reactive glia supports the idea that gliotic tissue is largely responsible for the failure of axonal regeneration in mammalian CNS.

Nerve fibre regeneration across the peripheral-central transitional zone

Neurons cannot negotiate an elongation across the peripheral (PNS)-central nervous system (CNS) transitional zone and grow into or out of the spinal cord in the mature mammal. The astrocytic rich CNS part of the spinal nerve root is most effective in preventing regeneration even of nerve fibres from transplanted embryonic ganglion cells. Regeneration of severed nerve fibres into the spinal cord occurs when the transition zone is absent as in the immature animal. Before the establishment of a transition zone there is also new growth of neuronal processes from dorsal horn neurons distally to the injured dorsal root. Thus the experimental strategy to reestablish spinal cord to peripheral nerve connectivity has been to delete the transitional region and implant severed ventral or dorsal roots into the spinal cord. Dorsal root implantation resulted in reestablished afferent connectivity by new neuronal processes from secondary sensory neurons in the dorsal horn of the spinal cord extending into the PNS. The ability for plasticity in these cells allowed for a concurrent retention of their original rostral projection. Ventral root implantation into the spinal cord corrected deficit motor function. In a long series of experiments performed in different species, the functional restitution was demonstrated to depend on an initial regrowth of motor neuron axons through spinal cord tissue (CNS). These findings have led to the design of a new surgical strategy in cases of traumatic spinal nerve root injuries.

Intercellular adhesion molecule-1 expression on glia following brain injury: participation of interleukin-1 beta

Interleukin-1 (IL-1), one of the most important inflammatory cytokines, promotes glia to express intercellular adhesion molecule-1 (ICAM-1) in vitro. IL-1 is known to be produced in situ immediately after brain insults and recently we have found that glia, including astrocytes, express ICAM-1 in vivo following cortical stab wounds. To evaluate the participation of IL-1 beta in posttraumatic ICAM-1 expression on glia in vivo, we performed the following experiments. A cortical stab wound was made in the brain of a mouse. ICAM-1-immunopositive glia began to emerge around the wound from 6 h postlesioning. The number of cells reached a maximum at 48 h and persisted until 7 days postlesioning. Next, a neutralizing monoclonal antibody against IL-1 beta was infused into the wound immediately following the injury. This treatment resulted in a significant reduction of ICAM-1-positive glia at 24 and 48 h postlesioning. We conclude, therefore, that IL-1 beta affects ICAM-1 expression on glia in vivo after experimental brain injury and presumably plays an important role in brain wound repair.

Angiotensin IV inhibits neurite outgrowth in cultured embryonic chicken sympathetic neurones

Angiotensin IV (Val-Tyr-Ile-His-Pro-Phe) is reported to enhance apomorphine induced stereotypy and to improve memory recall through actions on specific binding sites in the central nervous system. In the present study, 10 nM angiotensin IV or angiotensin II inhibited neurite outgrowth from cultured E11 chicken paravertebral sympathetic neurones by 25%. The effects of both peptides were inhibited by a 1 microM concentration of the angiotensin IV analogues. WSU 4042, Nle1-Y-I-amide or Nle1-AIV, but not by the avian angiotensin II antagonists, [Sar1,Ile8]Ang II or CGP 42112, suggesting that the inhibition of neurite outgrowth by both peptides is mediated by the angiotensin IV binding site. These results suggest that angiotensin IV may be involved in neurite modelling and may therefore have an important role in neuronal development.

Expression of insulin-like growth factor-1 (IGF-1) and IGF-binding protein 2 (IGF-BP2) in the hippocampus following cytotoxic lesion of the dentate gyrus

Receptor binding and gene expression of several members of the IGF gene family were examined in the rat brain following lesion of the hippocampal dentate gyrus granular cells by intradentate colchicine injection. Dentate granular cell loss was accompanied by extensive reactive gliosis in the lesioned hippocampus and damaged overlying cortex, as verified by the increase in GFAP mRNA and BS-1 lectin binding. At 4 days post-lesion, 125I-IGF-2 binding was dramatically increased within the lesioned dentate gyrus and damaged overlying cortex, and corresponded temporally and anatomically with increased IGF-BP2 gene expression following the lesion. Increased IGF-BP3 gene expression was only observed in the overlying cortex at 10 days post-lesion, and corresponded with an increase in 125I-IGF-1 binding at the injured surface of the cortex. Type-2 IGF receptor mRNA expression was reduced to background levels in the lesioned dentate gyrus, suggesting that IGF-BP2 was a major component of the observed increase in 125I-IGF-2 binding. In situ hybridization also revealed a prominent increase in IGF-1 mRNA expression by 4 days post-lesion, which was localized within the lesioned dentate gyrus and damaged cortical areas, and was shown to be expressed by microglia. While no IGF-2 mRNA expression was observed within the CNS, either prior to, or following the lesion, IGF-2 mRNA expression was observed in the choroid plexus, meningeal membranes, and in blood vessel endothelium, providing a potential source for the transport of IGF-2 into the CNS. In the injured CNS, increased IGF-BP2 expression may act to maintain or transport IGF-1 or IGF-2, as well as modulate the local autocrine and paracrine actions of the IGFs. Increased microglial IGF-1 expression following colchicine treatment correlates with the timing of a number of post-traumatic events within the CNS, suggesting that IGF-1 may have a role as a neuroprotectant for surviving neurons and signal for local neuronal sprouting, as well as a role in reactive astrogliosis.

A potent inhibitor of neurite outgrowth that predominates in the extracellular matrix of reactive astrocytes

In a model of astrogliosis in vitro, cultured cortical astrocytes were triggered into a functionally reactive state by an immobilized fragment of the beta-amyloid peptide. Induced astrocytes produced an extracellular matrix that inhibited the outgrowth of embryonic CNS axons. Within the extracellular matrix deposited by reactive astrocytes, we found an overall increase in the deposition of chondroitin sulphate that accounted for the inhibition. Specifically, we have detected an increased biosynthesis of a small chondroitin/heparan sulphate proteoglycan that is a potent inhibitor of axon outgrowth. We further suggest that this proteoglycan, or related molecules yet to be discovered, may play a role in gliosis-mediated regenerative failure of CNS axons.

Up-regulation of a keratan sulfate proteoglycan following cortical injury in neonatal rats

The up-regulation of the keratan sulfate proteoglycan (ABAKAN) was examined using indirect immunohistochemical methods. Previous studies indicate that the keratan sulfate proteoglycan is associated with astrocytes in the optic nerve and in the developing rat brain. In model culture systems, this proteoglycan is capable of inhibiting the growth of neurites over laminin. To determine whether the proteoglycan is up-regulated specifically during reactive gliosis, stab wounds were made in the cerebral cortex of early postnatal rats, and the up-regulation of the proteoglycan was related to the developmentally regulated gliotic response to injury. Following a stab wound in the cortex of the late postnatal rat, reactive gliosis was consistently observed along with an up-regulation of ABAKAN. When the cortex was injured on postnatal day 2, there was a variable gliotic response and considerable variation in the regulation of proteoglycan expression. Biochemical analysis revealed that ABAKAN is a large proteoglycan with multiple keratan sulfate side-chains, at least one chondroitin sulfate side-chain and at least one additional carbohydrate chain with a terminal 3-sulfoglucuronic acid. Taken together, these data demonstrate that the boundary proteoglycan ABAKAN is also associated with reactive gliosis during early postnatal development.

Microglial response to N-methyl-D-aspartate-mediated excitotoxicity in the immature rat brain

The intracerebral injection of N-methyl-D-aspartate (NMDA) has been proposed as a model for hypoxic-ischemic insult in the immature brain. In this light, the aim of this study was to describe the time course of the microglial reaction in the areas undergoing primary degeneration at the site of intracortical NMDA injection as well as in areas undergoing secondary anterograde and/or retrograde degeneration. Fifty nanomoles of NMDA were injected in the sensorimotor cortex of 6-day-old rats. After survival times ranging from 10 hours to 28 days, cryostat sections were stained for routine histology and for the demonstration of microglial cells by means of tomato lectin histochemistry. The areas affected by primary degeneration caused by the intracortical injection of NMDA were the neocortex, the hippocampus, and the rostral thalamus. Secondary degeneration (retrograde and anterograde) was observed in the ventrobasal complex of the thalamus. The cortical lesion also caused Wallerian degeneration of the cortical descending efferents as observed in the basilar pons. Microglial reactivity in all these areas was present at 10 hours postinjection and was restricted to the areas undergoing neuronal or axonal degeneration. Reactive microglial cells were stained intensely and showed a round or pseudopodic morphology. At 3 days, an apparent increase in the number of tomato lectin-positive cells was observed in the areas undergoing neuronal death. By 7 days after the injection, the lesion became nonprogressive, and by 14 and 28 days, microglial cells showed moderate lectin binding and a more ramified morphology.

Combined fetal neural transplantation and nerve growth factor infusion: effects on neurological outcome following fluid-percussion brain injury in the rat

This study was designed to evaluate the histological and behavioral impact of fetal neural transplantation with and without neurotrophin infusion in rats subjected to traumatic brain injury using a clinically relevant model of lateral fluid-percussion brain injury. Adult male Sprague-Dawley rats received lateral fluid-percussion brain injury of moderate severity (2.1-2.3 atm). Twenty-four hours after injury, minced fetal cortical grafts (E16) were stereotactically transplanted into the site of injury cavity formation (in 32 rats). Ten control animals received injections of saline. A third group of 29 animals that received transplants also underwent placement of a miniosmotic pump (immediately after transplantation) to continuously infuse nerve growth factor (NGF) directly into the region of graft placement for the duration of the experiment. A fourth group of eight animals underwent transplantation of fetal cortical cells that had been dissociated and placed in suspension. Animals were evaluated at 72 hours, 1 week, and 2 weeks after injury for cognitive function (using the Morris water maze), posttraumatic motor dysfunction, and transplant survival and morphology (using Nissl and modified Palmgren's silver staining techniques). Robust survival of whole-tissue transplants was seen in 65.5% of animals and was not increased in animals receiving NGF infusion. Animals receiving transplants of cell suspension had no surviving grafts. Brain-injured animals receiving transplants showed significant cognitive improvements compared with controls at the 2-week evaluation. Significantly improved memory scores were seen at all evaluation times in animals receiving both NGF and transplants compared with injured controls and compared with animals receiving transplants alone at the 72-hour and 1-week evaluations. Neurological motor function scores were significantly improved in animals receiving transplants alone and those receiving transplants with NGF infusion. Histological evaluation demonstrated differentiation of grafted cells, decreased glial scarring around transplants when compared with control animals, and the presence of neuronal fibers bridging the interface between graft and host. This study demonstrates that fetal cortical cells transplanted into the injured cortex of the adult rat can improve both posttraumatic cognitive and motor function and interact with the injured host brain.

Pharmacological treatment of central nervous system trauma

Traumatic injuries to the brain or spinal cord cause tissue damage, in part by initiating reactive biochemical changes. Pharmacological approaches aim to modify this delayed injury response by blocking one or more components of the reactive biochemical/metabolic cascade. This minireview summarizes both historical and recent developments in experimental and clinical treatment of CNS trauma. Potential treatments include: corticosteroids, antioxidants or free radical scavengers; drugs that modify arachidonic acid metabolism, platelet-activating factor antagonists; gangliosides; modulators of monoamine actions; opioid receptor antagonists; thyrotropin-releasing hormone and thyrotropin-releasing hormone analogues; glutamase receptor antagonists; calcium channel blockers; agents that modify the inflammatory/immune response; and trophic factors. Understanding the mechanisms of action for these compounds can permit rational drug development/application, delineation of the therapeutic window, and laying of the ground-work for evaluating potential synergistic effects of combination treatment strategies.

Olfactory bulb ensheathing glia: a unique cell type with axonal growth-promoting properties

The olfactory bulb (OB) is a structure of the central nervous system (CNS) in which axonal growth occurs throughout the lifetime of the organism. A major difference between the OB and the remaining CNS is the presence of ensheathing glia in the first two layers of the OB. Ensheathing glia display properties that might be involved in the process of regeneration and they appear to be responsible for the permissibility of the adult OB to axonal growth. In fact, transplants of ensheathing glia can be used as promoters of axonal regeneration within the adult CNS. The axonal growth-promoting properties of ensheathing glia make the study of this cell type interesting for understanding the mechanisms underlying axonal regeneration. Several groups have studied OB ensheathing cells extensively in an attempt to classify them within any of the known glial groups. However, this cell type does not exhibit the phenotypic features of any glial population described thus far. In this article we review the characteristics that differentiate ensheathing glia from other peripheral and central glial populations as well as the properties that involve them in axonal regeneration. The evidence suggests that ensheathing glia are unique, have their own identity, and do not belong to any previously described glial type.

Expression of glial fibrillary acidic protein in areas of focal cerebral ischemia accompanies neuronal expression of 72-kDa heat shock protein

We examined the astrocytic GFAP and neuronal HSP-72 responses to transient middle cerebral artery (MCA) occlusion in the rat. Three groups of rats (n = 79) were studied: (1) fixed duration of MCA occlusion (120 min) and variable durations of reperfusion (0.5, 3, 6, 9, 12, 24, 48, 96 and 168 h); (2) variable durations of MCA occlusion (10, 20, 30, 60, 90, and 120 min) and a fixed duration of reperfusion (48 h); and (3) controls: sham operated rats and normal rats. Coronal sections from each brain were reacted with appropriate antibodies to GFAP and HSP-72 and stained with H&E for evaluation of cellular response to ischemia. Our data show that after MCA occlusion: (1) GFAP expression was found in the boundary zone to the infarct or in areas of selective incomplete ischemic necrosis; (2) GFAP expression was localized to the same areas where neurons express HSP-72 and are destined to survive the ischemic insult; and (3) HSP-72 expression was not found in astrocytes in any of the experimental groups. These studies suggest that after transient focal ischemia in the rat: areas where both GFAP and HSP-72 expression are lost are destined to become necrotic, even though cells may appear morphologically intact in the H&E preparations, and expression of GFAP and HSP-72 reflects astrocytic and neuronal viability, respectively.

In vivo induction of the growth associated protein GAP43/B-50 in rat astrocytes following transient middle cerebral artery occlusion

Immunohistochemistry was used to investigate the induction of growth-associated protein GAP43/B-50 in the astrocytes of rat cerebrum in vivo following ischemic injury produced by 30 min of transient middle cerebral artery occlusion. Three days after operation, GAP43 immunoreactivity first appeared in some astrocytic populations surrounding the infarcted lesion. Induction of GAP43 in those astrocytes persisted for up to 14 days and disappeared at 30 days postoperation. Double-immunofluorescence staining confirmed that the GAP43-immunoreactive astrocytes examined were all positive for glial fibrillary acidic protein. Our present data suggest that certain astrocytes could be induced to synthesize GAP43 in vivo in response to an ischemic insult in adult rats.

Differential effects of glycosaminoglycans on neurite outgrowth from hippocampal and thalamic neurones

Chondroitin sulphate proteoglycans are expressed in a temporally restricted pattern from embryonic day 17 to postnatal day 0 in both the thalamus and the cortical subplate, to which thalamic neurones transiently project. To study whether chondroitin sulphate proteoglycans could be specifically involved in the modulation of thalamic axon outgrowth, we compared neurite outgrowth from cultured rat embryonic hippocampal and thalamic neurones, in the presence of chondroitin sulphate type C (isolated from shark cartilage) and chondroitin sulphate type B (dermatan sulphate; isolated from bovine mucosa). When added to the culture medium, both types of glycosaminoglycan lowered the adhesion to laminin and polylysine of both hippocampal and thalamic neurones. However, only chondroitin sulphate specifically modified the pattern of thalamic but not hippocampal neurone outgrowth, promoting axon growth. The morphological changes induced by chondroitin sulphate were concentration dependent and correlated with the selective binding of chondroitin sulphate to the neuronal plasma membrane and its subsequent internalisation. Chondroitin sulphate loosely bound to the surface of hippocampal neurones, but was not internalised. These results indicate that proteoglycans, and in particular the glycosaminoglycan component of these molecules, can differentially modulate neurite outgrowth, depending on their biochemical composition and on the type of neurones they bind to; this would be a possible mechanism of controlling axon guidance in vivo.