Effects of transforming growth factor beta 1 on scar production in the injured central nervous system of the rat

Inflammatory cytokines interact to modulate extracellular matrix and astrocytic support of neurite outgrowth

Following injury to the central nervous system, an astroglial scar forms that is thought to impede neuronal regeneration and recovery of function. It is our hypothesis that inflammatory cytokines act upon astrocytes to alter their biochemical and physical properties, which may in turn be responsible for failed neuronal regeneration. We have therefore examined the interactions of two cytokines with prominent actions following injury, interferon-gamma (IFN-gamma) and basic fibroblast growth factor (FGF2), in modulating the extracellular matrix and proliferation of astrocytes in culture. We also evaluated the effects of these cytokines on the ability of astrocytes to support the growth of neurites. IFN-gamma significantly inhibited the proliferation of rat cortical astrocytes both in serum-free and serum-containing media as measured by [3H]thymidine incorporation. Furthermore, IFN-gamma also antagonized FGF2-induced proliferation. In parallel, IFN-gamma reduced the levels of the ECM molecules tenascin, laminin, and fibronectin as evaluated by Western blot analysis and immunocytochemistry. Similarly, IFN-gamma also antagonized FGF2-induced tenascin formation. While IFN-gamma-pretreated astrocyte monolayers did not differ from control in their ability to support neurite outgrowth of cortical neurons, it antagonized the enhancement of neurite outgrowth on FGF2-treated monolayers. We demonstrate that IFN-gamma did not alter signal transduction through the FGF2 receptor down to the phosphorylation of mitogen-activated protein kinase, suggesting that the interaction is at the level of transcriptional regulation or that an alternate pathway is involved. These results support the hypothesis that inflammatory cytokines interact to modulate several facets of the gliotic response and such interactions may be important in creating the biochemical and physical properties of the glial scar.

Activated macrophage/microglial cells can promote the regeneration of sensory axons into the injured spinal cord

A prominent role for phagocytic cells in the regenerative response to CNS or PNS injury has been suggested by numerous studies. In the present work we tested whether increasing the presence of phagocytic cells at a spinal cord injury site could enhance the regeneration of sensory axons from cut dorsal roots. Nitrocellulose membranes treated with TGF-beta or coated with microglial cells were cotransplanted with fetal spinal cord tissue into an injured adult rat spinal cord. Cut dorsal roots were apposed to both sides of the nitrocellulose. Four weeks later, animals were sacrificed and spinal cord tissue sections were processed for immunocytochemical detection of calcitonin gene-related peptide (CGRP-ir) to identify regenerated sensory axons. Adjacent sections were processed with the antibody ED-1 or the lectin GSA-B4 for detection of macrophage/microglial cells in association with the regrowing axons. Qualitative and quantitative data indicate a correlation between the pattern and extent of axonal regeneration and the presence of phagocytic cells along the nitrocellulose implant. Axonal regeneration could be experimentally limited by implanting a nitrocellulose strip treated with macrophage inhibitory factor. These results indicate that increasing the presence of activated macrophage/microglial cells at a spinal cord injury site can provide an environment beneficial to the promotion of regeneration of sensory axons, possibly by the release of cytokines and interaction with other nonneuronal cells in the immediate vicinity.

Reactive astrocytes: cellular and molecular cues to biological function

For several decades, the reactive gliosis that occurs after an injury to the CNS has been considered one of the major impediments to axonal regeneration. Nevertheless, recent studies have suggested that in certain conditions, reactive astrocytes may provide a permissive substratum to support axonal regrowth. The important criteria, allowing for the distinction between permissive and non-permissive gliosis, are the ultrastructural 3D organization of the scar and more importantly the recognition molecules expressed by reactive astrocytes. Reactive astrocytes express surface molecules and produce various neurotrophic factors and cytokines. The latter in turn might modulate the production of recognition molecules by reactive astrocytes, allowing them to support post-lesional axonal regrowth. Although numerous recent articles have focused on cytokines and cell adhesion molecules, scant attention has been paid to reactive astrocytes. Reactive astrocytes should be considered a key element, like neurons, of a dynamic environment, thus forming with neurons a functional unit involved in homeostasis, plasticity and neurotransmission. Attempts are in progress to identify molecular markers for reactive astrocytes.

Leukemia inhibitory factor is expressed in astrocytes following cortical brain injury

The neuropoietic cytokine leukemia inhibitory factor (LIF) can act as a trophic factor, enhancing neuronal survival, and as a differentiation factor altering neuronal and glial gene expression. LIF also plays a role in the response to injury of the peripheral nervous system, as indicated by an increase in the amount of its mRNA within nonneuronal injury response in LIF knock-out mice. To determine if LIF is regulated after injury to the central nervous system, we surgically lesioned the cortex in adult rat brain. Using a quantitative RNAse protection assay, we find that LIF mRNA increases 30-fold following injury. The amount of this transcript goes up within 6 h after injury, reaches a peak at 24 h and returns to baseline by 7 days postlesion. In situ hybridization analysis reveals LIF transcript-containing cells scattered throughout the ipsilateral cortex close, but not immediately adjacent to the lesion site. Double-labeling with a variety of antibodies reveals that LIF mRNA is induced in GFAP-positive astrocytes as well as in a small number of microglial cells. The striking induction of LIF transcripts in glia suggests that this cytokine may play a key injury-response role in the CNS as it does in the PNS, where LIF has been demonstrated to regulate neuropeptide expression both in vivo and in vitro.

Effects of pro-inflammatory cytokines in experimental spinal cord injury

Following injury to the spinal cord, secondary tissue damage leading to massive additional tissue loss and inflammatory reactions as well as scar formation takes place. The precise functions and effects of the inflammatory cells and their secreted factors are largely unclear. The present study investigates whether the exogenous local administration of pro-inflammatory cytokines to mice after spinal cord injury can influence these intrinsic processes. A mixture of murine recombinant interleukin-1beta (IL-1beta), interleukin-6 (IL-6) and tumour necrosis factor alpha (TNF alpha) was administered to the lesioned spinal cord of adult mice. These cytokines provoked an increased recruitment and activation of macrophages and microglial cells in the lesion area when administered 1 day post lesion. In contrast, when administered 4 days after the lesion, recruitment of macrophages was slightly increased while activation of microglia was decreased as compared to controls. The amount of tissue loss 7 days after trauma was smaller in the animals receiving the cytokine mixture than in the mice receiving Ringer control solution on day 4 after lesion. Thus the role of the inflammatory response in spinal cord injury seems to be complex and well regulated. Anti-inflammatory cytokines and factors probably also contribute to the outcome of the damage following injury to the spinal cord.

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.

Glial lineages and myelination in the central nervous system

Oligodendrocytes, derived from stem cell precursors which arise in subventricular zones of the developing central nervous system, have as their specialist role the synthesis and maintenance of myelin. Astrocytes contribute to the cellular architecture of the central nervous system and act as a source of growth factors and cytokines; microglia are bone-marrow derived macrophages which function as primary immunocompetent cells in the central nervous system. Myelination depends on the establishment of stable relationships between each differentiated oligodendrocyte and short segments of several neighbouring axons. There is growing evidence, especially from studies of glial cell implantation, that oligodendrocyte precursors persist in the adult nervous system and provide a limited capacity for the restoration of structure and function in myelinated pathways damaged by injury or disease.

Therapeutic approaches to organ fibrosis

Scarring of superficial tissues and chronic fibroses of major organs present major medical problems ranging from disfigurement to progressive disability and death. Growing understanding of the cellular and molecular events, which are common to these intractable disorders, now provides a favourable basis for the development of effective drug therapies. Much attention is focussed on the roles of the many cytokines and growth factors, which contribute to the fibrogenic process. The transforming growth factor (TGF)-beta 1 and 2 isoforms are among the most significant of these and approaches to control their activity include blocking the activation of latent TGF-beta, preventing the ligand-receptor interactions and the inhibition of down-stream signal transduction. Concerns regarding possible risks of the long-term suppression of TGF-beta function point to connective tissue growth factor (CTGF) as a possible alternative target. CTGF is induced by and appears to mediate at least some of the fibrogenic actions of TGF-beta, although not its important antimitogenic activity on epithelial cells. The fibrogenic effects of endothelins and angiotensin II have aroused considerable interest in the anti-fibrotic potential of antihypertensive agents designed primarily to limit the vasoconstrictive activities of these peptides. Polypeptides including interferons alpha and gamma, relaxin, TGF-beta 3 and hepatocyte growth factor, all show an ability to limit fibrogenesis in either clinical or experimental situations. Finally, inhibitors of the enzymes required for the post-translational processing of collagens, including prolyl 4-hydroxylase, C-proteinase and lysyl oxidase provide a more direct means of reducing the deposition of fibrillar collagens into the extracellular matrix although the potentially adverse effects of sustained manipulation of collagen metabolism remain to be investigated.

Transforming growth factor beta s and wound healing

The Transforming Growth Factor beta superfamily (TGF beta) is one of the most complex groups of cytokines with widespread effects on many aspects of growth and development. The TGF beta isoforms and other family members, e.g. Activins and BMPs, have diverse effects in similar physiological situations. TGF beta is involved in the wound healing process. The three mammalian isoforms (TGF beta 1, 2 and 3) and recently other family members, e.g. Activin, have been localised in healing wounds. Manipulation of the ratios of TGF beta superfamily members, particularly the ratio of TGF beta 1 relative to TGF beta 3, reduces scarring and fibrosis. Such manipulations include reducing the levels of TGF beta 1/TGF beta 2 using neutralising antibodies or preventing the activation of TGF beta s. In chronic or impaired wounds the exogenous addition of TGF beta superfamily members accelerates aspects of the healing process. This review summarises evidence for the role of TGF beta superfamily members in wound healing and how modulation of TGF beta levels can prevent scarring and fibrosis.

Reconstruction of the glial environment of a photochemically induced lesion in the rat spinal cord by transplantation of mixed glial cells

It is becoming increasingly apparent that the astrocytic environment is critical to the normal development and functioning of the CNS, and that acute injury to the spinal cord causes destruction of glial cells in addition to neurones and axons. The aims of this study were to assess the viability of reconstructing the astrocytic environment of a cystic spinal cord lesion by transplantation of glial cells and to examine the effect of the transplanted cells on meningeal cell invasion and revascularisation of the lesion and on axonal regeneration. Neonatal rat and kitten mixed glial cells and the CG-4 rat O-2A progenitor cell line were transplanted into a lesion produced in the dorsal funiculus of the rat spinal cord by photochemical infarction. The animals were killed 4 weeks after injury, their cords examined with light and electron microscopy and compared with control animals that were injected with medium alone. Transplantation of all three preparations resulted in increased numbers of astrocytes in the area of Wallerian degeneration cranial to the lesion and within the cyst. Mixed glial cell cultures prepared from neonatal rat forebrain contained cells with in vitro characteristics of type-1 astrocytes, and produced dense clusters of astrocytes that were surrounded by meningeal cells, resulting in a fragmented environment in the cyst. In contrast, glial cell cultures prepared from kitten forebrain and the CG-4 cell line produced cells that filled the cyst with a loose network of fine processes and reduced meningeal cell infiltration of the lesion. The CG-4 cell line significantly increased the density of blood vessels in the centre of the lesion and the number of spared axons present dorsal to the lesion, but none of the preparations significantly increased the number of axons regenerating at the caudal end of the lesion. We conclude that O-2A progenitor-derived astrocytes are more suitable for reconstruction of the glial environment of a cystic lesion in the rat spinal cord than 'type-1 like' astrocytes and would therefore be the cell of choice to engineer to produce factors that promote axonal regeneration.

Transforming growth factor-beta 1 promotes re-elongation of injured axons of cultured rat hippocampal neurons

Transforming growth factor-beta 1 (TGF-beta 1) is highly expressed in neural tissues following brain injuries. To determine a role of TGF-beta 1 in neural pathologies, axons of cultured rat hippocampal neurons were cut by local irradiation of laser beam, and the effect of TGF-beta 1 on neurite regeneration following axotomy was investigated. The axonal growth was stopped by laser irradiation, but addition of TGF-beta 1 remarkably promoted the axonal re-elongation from the injured site. The effect of TGF-beta 1 was concentration dependent and seen maximally at a concentration of 1 ng/ml. These results suggest that TGF-beta 1 has a capability of promoting axonal regeneration of brain neurons after lesioning.

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.

Regulation of thrombospondin in the regenerating mouse facial motor nucleus

Thrombospondin (TSP) is a multifunctional extracellular matrix protein that plays a role in neuronal migration and axonal outgrowth in the developing central nervous system. In the current study we have examined the localization and regulation of TSP immunoreactivity (TSP-IR) during neuronal regeneration in the axotomized facial motor nucleus using Western blotting and light and electron microscopy. Transection of the facial nerve led to a gradual increase in TSP-IR in the regenerating motoneurons, peaking 4-7 days after injury (DAI). In addition to regenerating neurons, axotomy also caused a rapid upregulation of TSP-IR on activated microglia throughout the facial nucleus, with a maximum of 2-3 DAI, and a second increase at 14-21 DAI on microglial aggregates surrounding degenerating motoneurons and in neuronophagic microglia. In summary, injury leads to the induction of thrombospondin on axotomized neurons and activated microglia, peaking at the times of maximal posttraumatic microglial proliferation and during neuronal phagocytosis. Since thrombospondin is a multimodal extracellular matrix protein with a variety of cell attachment sites, thrombospondin might serve to link microglia and injured neurons, followed by microglial proliferation and removal of the neuronal debris.

Inflammation in EAE: role of chemokine/cytokine expression by resident and infiltrating cells

Experimental allergic encephalomyelitis (EAE) is an inflammatory demyelinating disease of the central nervous system (CNS) which has many clinical and pathological features in common with multiple sclerosis (MS). Comparison of the histopathology of EAE and MS reveals a close similarity suggesting that these two diseases share common pathogenetic mechanisms. Immunologic processes are widely accepted to contribute to the initiation and continuation of the diseases and recent studies have indicated that microglia, astrocytes and the infiltrating immune cells have separate roles in the pathogenesis of the MS lesion. The role of cytokines as important regulatory elements in these immune processes has been well established in EAE and the presence of cytokines in cells at the edge of MS lesions has also been observed. However, the role of chemokines in the initial inflammatory process as well as in the unique demyelinating event associated with MS and EAE has only recently been examined. A few studies have detected the transient presence of selected chemokines at the earliest sign of leukocyte infiltration of CNS tissue and have suggested astrocytes as their cellular source. Based on these studies, chemokines have been postulated as a promising target for future therapy of CNS inflammation. This review summarized the events that occur during the inflammatory process in EAE and discusses the roles of cytokine and chemokine expression by the resident and infiltrating cells participating in the process.

Rodent models of Alzheimer's disease: rat A beta infusion approaches to amyloid deposits

The development of rodent models for Alzheimer's disease is a critical step for both understanding the disease and developing therapeutic drugs. Transgenic and knockout mouse models will elucidate some important aspects of the etiology of the disease and the development of pharmaceutical treatments. Here, we will focus on the advantages of nontransgenic models. In nontransgenic rat models, intraventricular infusion of A beta 1-40 (alone) generally results in diffuse deposition of A beta with very few focal plaque-like amyloid deposits after a 30-day intraventricular infusion. However, we have recently found that large numbers of scattered A beta immunoreactive plaque-like deposits can be produced in retired female Sprague-Dawley rat breeders using intraventricular infusion of A beta combined with neuropil injection of transforming growth factor beta 1(TGF beta). A beta that was not associated with the large deposits was often immunolocalized with neurons and cell processes. Immunogold electron microscopy demonstrated the presence of A beta in endosome/lysosomes of neuronal processes and glia and basal lamina. In some cases this labeling was clearly in lysosomes of degenerating neurites. This model allows one to introduce A beta and other plaque-associated factors without overexpression of potentially confounding APP domains. We conclude that A beta infusion models will be a useful complement to transgenic approaches to Alzheimer's pathology.

Effects of exogenous transforming growth factor-beta 1 on spinal cord injury in rats

This study was undertaken to examine the effect of transforming growth factor-beta 1 (TGF-beta 1) administered into the subarachnoid space after spinal cord injury (SCI) on the increased production of inducible-nitric oxide synthase (i-NOS) in the injured spinal cord in rats. The expression of i-NOS mRNA after SCI was remarkably down-regulated by TGF-beta 1 in vivo. Rats treated with TGF-beta 1 showed a better outcome regarding hindlimb motor dysfunction in the first 5 days after injury compared to the saline-treated rats. However, the final outcome was not better and fibrous scar formation in the injured spinal cord was more evident, which was demonstrated as increased immunoreactivity of fibronectin in the later stage after SCI. These results provide evidence of both positive and negative contributions of TGF-beta 1 to the pathology associated with SCI.

Cocultures of meningeal and astrocytic cells--a model for the formation of the glial-limiting membrane

The glial-limiting membrane at the border of the central nervous system (CNS) consists of glial endfeet covered by a basal lamina. The formation of the glia limitans seems to be controlled by adjacent meninges but only little is known about this interaction. In the present study astrocytes and meningeal cells were investigated in vitro to see if cocultures of these cells can serve as a suitable model for the differentiation of the glial-limiting membrane and can be used to define the conditions under which the glial-limiting membrane develops. The following observations were made in cocultures of meningeal and astrocytic cells of two-day-old rats: (i) epithelioid astrocytes were transformed into stellate cells; (ii) single colonies of proliferating epithelioid astrocytes were generated; (iii) the area around these colonies becomes devoid of meningeal cells, which seem to form a circular border around the astroglial islands; (iv) from the glial colonies long thin glial processes grow towards the surrounding meningeal cells, terminating at the site of contact; (v) in the contact zone between meningeal cells and astrocytes irregular shaped deposits of electron dense material resembling a basal lamina were seen. These observations indicate that indeed a structure resembling a glial-limiting membrane develops in cocultures of meningeal and astrocytic cells. Its formation depends on the balance of growth promoting effects of meningeal cells on astrocytes and growth inhibiting effects of astrocytes on meningeal cells. Both activities can be enriched from conditioned media of pure astrocytic or meningeal cell culture. The proposed model of meningo-astrocytic cocultures may be a helpful instrument for further investigations on the formation of the glia limitans.

Release and activation of platelet latent TGF-beta in blood clots during dissolution with plasmin

Transforming growth factor beta 1 (TGF-beta 1) is a platelet-derived cytokine involved in both normal wound healing and scarring. We show that human platelets contain two pools of latent TGF-beta 1, which constitute more than 95% of the total TGF-beta assayed in whole platelets. During clotting, one pool, the large latent TGF-beta complex consisting of latent TGF-beta binding protein (LTBP), the latency-associated peptide (LAP) and the 25-kD mature TGF-beta 1 dimer is released into the serum. A second pool, which contains LAP but not LTBP, is retained in the clot, but can be released by RGD peptide. When the clot is dissolved by plasmin this bound TGF-beta 1 is gradually activated and released. If similar mechanisms operate in vivo, the clot will act as a slow-release capsule of TGF-beta 1 activity during wound healing.

Expression of transforming growth factor-beta 1 in dystrophic patient muscles correlates with fibrosis. Pathogenetic role of a fibrogenic cytokine

Duchenne muscular dystrophy is a fatal disorder characterized by progressive muscular weakness, wasting, and severe muscle contractures in later disease stages. Muscle biopsy reveals conspicuous myofiber degeneration and fibrosis substituting muscle tissue. We quantitatively determined mRNA of the potent fibrogenic cytokine transforming growth factor-beta 1 by quantitative PCR in 15 Duchenne muscular dystrophy, 13 Becker muscular dystrophy, 11 spinal muscular atrophy patients, and 16 controls. Higher transforming growth factor-beta 1 expression was greater in Duchenne muscular dystrophy patients than controls (P = 0.012) and Becker patients (P = 0.03). Fibrosis was significantly more prominent in Duchenne muscular dystrophy than Becker muscular dystrophy, spinal muscular atrophy, and controls. The proportion of connective tissue in muscle biopsies increased progressively with age in Duchenne muscular dystrophy patients, while transforming growth factor-beta 1 levels peaked at 2 and 6 yr of age. Transforming growth factor-beta 1 protein was also detected by immunocytochemistry and immunoblotting. Our findings suggest that transforming growth factor-beta 1 stimulates fibrosis in Duchenne muscular dystrophy. Expression of transforming growth factor-beta 1 in the early stages of Duchenne muscular dystrophy may be critical in initiating muscle fibrosis and antifibrosis treatment could slow progression of the disease, increasing the utility of gene therapy.

Transforming growth factor-beta 1: a lesion-associated cytokine of the nervous system

Lesions to the nervous system are nearly universally accompanied by a glial response involving both microglia and astrocytes. The growth and immunoregulatory cytokine transforming growth factor-beta 1 (TGF-beta 1) has potent effects on glial cells in vitro and may play a role in regulating glial activation in vivo. Though present only at very low levels in the normal brain, TGF-beta 1 mRNA is strongly upregulated in a number of different experimental models suitable to study glial responses. Following axotomy of the facial nerve of the rat, about a three-fold increase of TGF-beta 1 mRNA in the regenerating nucleus was observed with a time-course closely matching that of glial activation. Putative activated microglial cells are the major cellular source as revealed by in-situ hybridization. TGF-beta 1 was also found to be upregulated around brain tumors, in the spinal cord in response to peripheral nerve inflammation and in the postischemic hippocampus. In all systems investigated, TGF-beta 1 mRNA could be localized predominantly to cells with the typical nuclear morphology of microglia. In the peripheral nervous system, nerve transection leads to a massive increase in TGF-beta mRNA expression both proximal and distal to the cut site. However, whereas TGF-beta 1 mRNA is restricted to the nerve stump in the proximal segment, expression is diffuse and widespread throughout the denervated distal segment where it was localized mainly to cells with macrophage morphology.(ABSTRACT TRUNCATED AT 250 WORDS)

The ependyma: a protective barrier between brain and cerebrospinal fluid

This review summarizes the current scientific literature concerning the ependymal lining of the cerebral ventricles of the brain with an emphasis on selective barrier function and protective roles for the common ependymal cell. Topics covered include the development, morphology, protein and enzyme expression including reactive changes, and pathology. Some cells lining the neural tube are committed at an early stage to becoming ependymal cells. They serve a secretory function and perhaps act as a cellular/axonal guidance system, particularly during fetal development. In the mature mammalian brain ependymal cells possess the structural and enzymatic characteristics necessary for scavenging and detoxifying a wide variety of substances in the CSF, thus forming a metabolic barrier at the brain-CSF interface.

Transforming growth factor beta 1 and parathyroid hormone-related protein control the secretion of dipeptidyl peptidase II by rat astrocytes

Meninges synthesize parathyroid hormone-related protein (PTHrP) and transforming growth factor (TGF beta 1). Both factors control the activity of the glial enzyme dipeptidyl peptidase II (DPP II): TGF beta 1 induces the secretion of enzyme activity by cultivated astrocytes in a dose dependent manner. The maximal effect is achieved with a concentration of 10 ng/ml and is dependent on the time of incubation. PTHrP itself has no effect on the release of DPP II activity but considerably reduces the effect of TGF beta 1. It is assumed that DPP II influences the course of cicatrization after penetrating injuries of the brain and thus, meninges control glial scarring by the release of TGF beta 1 and PTHrP.

Demonstration of parathyroid hormone-related protein in meninges and its receptor in astrocytes: evidence for a paracrine meningo-astrocytic loop

In contrast to the nervous and glial tissue of the adult rat brain the meninges are immunoreactive for parathyroid hormone-related protein (PTHrP), a hormone that binds with high affinity to the recently cloned PTH/PTHrP receptor. Accordingly immunoreactivity is found in cultured meningeal cells but not in astrocytes. In contrast, astrocytes but not meningeal cells synthesize the mRNA for the PTHrP receptor shown by reverse transcription of total RNA preparations and subsequent polymerase chain reaction with primers specific for the PTHrP receptor. The expression of the PTH/PTHrP receptor was confirmed by the dose-dependent activation of the adenylate cyclase in astrocytes and the rapid development of cellular processes following on incubation with PTHrP. We conclude that PTHrP secreted by meninges forms a paracrine meningo-astrocytic loop and may cause astrocytic differentiation, possibly involved in the formation of the glial limiting membrane.

Spinal axons in central nervous system scar tissue are closely related to laminin-immunoreactive astrocytes

Although transected central nervous system axons fail to regrow after injuries in adult mammals, they send sprouts into the scar tissue that forms at the lesion. We have investigated the relation between scar cells, laminin-like immunoreactivity and cut spinal axons in two previously characterized spinal cord lesion types. Labeling with antisera to glial fibrillary acidic protein and laminin demonstrated that the scar tissue formed after lesions in the rat and cat dorsal and ventral funiculi showed prominent gliosis and strong laminin-like immunoreactivity four days to one year postlesion. Axonal sprouts in the scar, visualized with antibodies to neurofilament (RT97) or by tracing using fluorescein-conjugated dextran, were ensheathed by a thin layer of strongly laminin-immunoreactive tissue. Immunoelectron microscopy demonstrated that axons in the scar were ensheathed predominantly by astrocytes, and that the surface of the cells outlining the axons in the scar showed strong laminin-like immunoreactivity. Adhesive and neurite orienting properties in the scar tissue were assessed in an in vitro system where PC12 cells were cultured on spinal cord slices from dorsal funiculus-lesioned rats. Very few cells adhered to the spinal cord section except for the part where the scar tissue had formed, where numerous cells were attached. The PC12 cells that had adhered to the scar tissue were mainly seen in parts of the scar that showed laminin-like immunoreactivity and their neurites predominantly followed tissue showing laminin-like immunoreactivity. The close association between axonal sprouts and laminin-like immunoreactivity indicates a role for laminin in axonal growth and/or guidance in the injured spinal cord.

Growth factors in CNS repair and regeneration

Traumatic central nervous system (CNS) injury is a significant clinical problem in the developed world. After injuries that penetrate into either the mature brain or spinal cord, damaged neurons initially begin to regrow, but this regeneration is aborted as a fibrotic scar is laid down within the wound. Reconnection of several neuronal pathways does not occur. Functional recovery from such injuries is therefore poor and morbidity severe, particularly for those patients with spinal cord damage. Although palliative measures are available to improve the quality of life, there is no accepted treatment to restore impaired sensory or motor function, so patients remain significantly and permanently debilitated. However, the rapid recent advances that have been made in our understanding of the underlying cellular and trophic pathology of such injuries offer the potential for development of novel therapies to control scarring, enhance neuron survival and stimulate axon regeneration, thereby promoting functional recovery.