Matrix metalloproteases: degradation of the inhibitory environment of the transected optic nerve and the scar by regenerating axons

Optic nerve regeneration

Retinal ganglion cells are usually not able to regenerate their axons after optic nerve injury or degenerative disorders, resulting in lifelong visual loss. This situation can be partially reversed by activating the intrinsic growth state of retinal ganglion cells, maintaining their viability, and counteracting inhibitory signals in the extracellular environment. Advances during the past few years continue to extend the amount of regeneration that can be achieved in animal models. These findings give hope that clinically meaningful regeneration may become a reality within a few years if regenerating axons can be guided to their appropriate destinations.

Epidermal growth factor receptor antagonists and CNS axon regeneration: mechanisms and controversies

The reasons for the failure of central nervous system (CNS) axons to regenerate include the presence of myelin- and non-myelin derived inhibitory molecules, neuronal apoptosis and the absence of a potent neurotrophic stimulus. Transactivation of the epidermal growth factor receptor (EGFR) has been implicated in signalling inhibition of axon growth in the CNS. Small molecule EGFR inhibitors such as AG1478 and PD168393 promote CNS axon growth after optic nerve transection despite the presence of inhibitory molecules in the environment of the regenerating axon. However, our results demonstrate that phosphorylated EGFR (pEGFR) is not present on regenerating axons and that the majority of pEGFR is present in glia, suggesting that EGFR cannot play a direct intra-axonal role in signalling inhibition and thus disinhibited CNS axon growth must be indirectly mediated by glia. We argue that EGFR may not have a role in signalling axon growth inhibition since AG1478 and PD168393 promotes neuronal neurite outgrowth in CNS myelin-inhibited cultures after EGFR knockdown. This review discusses the current evidences for and against the involvement of EGFR in signalling myelin inhibition.

MMP-2 is involved in synaptic remodeling after cochlear lesion

Lesion-induced neuroplasticity, including fiber degeneration, axonal growth, and synaptogenesis, involves dynamical changes of the extracellular matrix. We discovered that the matrix metalloprotease-2 (MMP-2), a major actor in extracellular matrix recomposition, is changed in distribution and increased in amount in the ventral cochlear nucleus after unilateral cochlear ablation. There was a remarkable coincidence of MMP-2 accumulation and GAP-43 expression in time and space. We obtained evidence indicating that MMP-2 is delivered to regions of emerging GAP-43 positive synaptic endings by postsynaptic neurons as well as by adjoining astrocytes. These results indicate a major role of MMP-2 in lesion-induced remodeling of central auditory networks and suggest a cooperativity with GAP-43-directed axonal outgrowth and synaptogenesis.

A new role for TIMP-1 in modulating neurite outgrowth and morphology of cortical neurons

Background

Tissue inhibitor of metalloproteinases-1 (TIMP-1) displays pleiotropic activities, both dependent and independent of its inhibitory activity on matrix metalloproteinases (MMPs). In the central nervous system (CNS), TIMP-1 is strongly upregulated in reactive astrocytes and cortical neurons following excitotoxic/inflammatory stimuli, but no information exists on its effects on growth and morphology of cortical neurons.

Principal Findings

We found that 24 h incubation with recombinant TIMP-1 induced a 35% reduction in neurite length and significantly increased growth cones size and the number of F-actin rich microprocesses. TIMP-1 mediated reduction in neurite length affected both dendrites and axons after 48 h treatment. The effects on neurite length and morphology were not elicited by a mutated form of TIMP-1 inactive against MMP-1, -2 and -3, and still inhibitory for MMP-9, but were mimicked by a broad spectrum MMP inhibitor. MMP-9 was poorly expressed in developing cortical neurons, unlike MMP-2 which was present in growth cones and whose selective inhibition caused neurite length reductions similar to those induced by TIMP-1. Moreover, TIMP-1 mediated changes in cytoskeleton reorganisation were not accompanied by modifications in the expression levels of actin, βIII-tubulin, or microtubule assembly regulatory protein MAP2c. Transfection-mediated overexpression of TIMP-1 dramatically reduced neuritic arbour extension in the absence of detectable levels of released extracellular TIMP-1.

Conclusions

Altogether, TIMP-1 emerges as a modulator of neuronal outgrowth and morphology in a paracrine and autrocrine manner through the inhibition, at least in part, of MMP-2 and not MMP-9. These findings may help us understand the role of the MMP/TIMP system in post-lesion pre-scarring conditions.

Optic nerve and vitreal inflammation are both RGC neuroprotective but only the latter is RGC axogenic

Intravitreal inflammation, induced by either lens injury, or intravitreal injection of zymosan (IVZ), protects RGC from apoptosis and stimulates axon regeneration after optic nerve transection. Here, we investigate the differential effects of intra-optic nerve zymosan (ONZ) and IVZ injections on RGC neuroprotection and axogenesis. After both IVZ and ONZ injection, zymosan-induced inflammation promoted a similar 4-/5-fold enhancement in RGC survival, compared to optic nerve transected controls, but only IVZ promoted RGC axon regeneration. IVZ was the most effective in activating retinal astrocyte/Müller cells while regulated intramembraneous proteolysis (RIP) of p75(NTR) and inactivation of Rho (key components of the axon growth inhibitory signalling cascade) occurred in both ONZ and IVZ, but only in the latter did RGC axons regenerate. We suggest that neuroprotective factors may be transported to RGC somata by retrograde transport after ONZ and diffuse into the retina after IVZ injection, but an axogenic agent is required to initiate and maintain disinhibited RGC axon regeneration that may be an exclusive property of a Müller cell-derived factor released after IVZ.

Friend or foe? Resolving the impact of glial responses in glaucoma

Glaucomatous vision loss results from the progressive degeneration of optic nerve axons and the death of retinal ganglion cells. This process is accompanied by dramatic alterations in the functional properties and distribution of glial cells in both the retina and the optic nerve head in a reaction commonly referred to as glial activation. The recent availability of rodent and cell culture glaucoma models has substantially contributed to our knowledge of glial activation under glaucomatous conditions. Conclusions drawn from these studies have led to the refinement of existing hypotheses and the generation of new ones. Because these hypotheses encompass both protective and injurious roles for glia, the impact of specific aspects of glial activation are current topics of intensive research, speculation, and debate in the field. With these unresolved issues in mind, this review will summarize recent progress in our understanding of the process of glial activation in the glaucomatous optic nerve head and retina.

Epidermal growth factor receptor inhibitors promote CNS axon growth through off-target effects on glia

Administration of epidermal growth factor receptor (EGFR) inhibitors (e.g. AG1478/PD168393) promotes central nervous system (CNS) axon regeneration in vivo by an unknown mechanism. Here, we show that EGFR activation is not required for AG1478-/PD168393-induced neurite outgrowth in cultures of dorsal root ganglion neurons (DRGN) with added inhibitory CNS myelin extract (CME), but is mediated by the paracrine and autocrine actions of the glia-/neuron-derived neurotrophins (NT) NGF, BDNF and NT-3 through Trk signalling in DRGN potentiated by elevated cAMP levels. The DRGN neurite growth seen in CME-inhibited cultures treated with AG1478 is eradicated by blocking Trk signalling but undiminished after siRNA knockdown of >90% EGFR. Moreover, addition of the combined triplet of NT restores neurite outgrowth in CME-inhibited cultures, when cAMP levels are raised. Accordingly, we suggest that chemical EGFR inhibitors act independently of EGFR, inducing glia and neurons to secrete NT and raising cAMP levels in DRG cultures, leading to Trk-dependent disinhibited DRGN neurite outgrowth.

High resolution in situ zymography reveals matrix metalloproteinase activity at glutamatergic synapses

Synaptic plasticity involves remodeling of extracellular matrix. This is mediated, in part, by enzymes of the matrix metalloproteinase (MMP) family, in particular by gelatinase MMP-9. Accordingly, there is a need of developing methods to visualize gelatinolytic activity at the level of individual synapses, especially in the context of neurotransmitters receptors. Here we present a high-resolution fluorescent in situ zymography (ISZ), performed in thin sections of the alcohol-fixed and polyester wax-embedded brain tissue of the rat (Rattus norvegicus), which is superior to the current ISZ protocols. The method allows visualization of structural details up to the resolution-limit of light microscopy, in conjunction with immunofluorescent labeling. We used this technique to visualize and quantify gelatinolytic activity at the synapses in control and seizure-affected rat brain. In particular, we demonstrated, for the first time, frequent colocalization of gelatinase(s) with synaptic N-methyl-D-aspartic acid (NMDA)- and AMPA-type glutamate receptors. We believe that our method represents a valuable tool to study extracellular proteolytic processes at the synapses, it could be used, as well, to investigate proteinase involvement in a range of physiological and pathological phenomena in the nervous system.

A novel role for PECAM-1 (CD31) in regulating haematopoietic progenitor cell compartmentalization between the peripheral blood and bone marrow

Although the expression of PECAM-1 (CD31) on vascular and haematopoietic cells within the bone marrow microenvironment has been recognized for some time, its physiological role within this niche remains unexplored. In this study we show that PECAM-1 influences steady state hematopoietic stem cell (HSC) progenitor numbers in the peripheral blood but not the bone marrow compartment. PECAM-1^−/− mice have higher levels of HSC progenitors in the blood compared to their littermate controls. We show that PECAM-1 is required on both progenitors and bone marrow vascular cells in order for efficient transition between the blood and bone marrow to occur. We have identified key roles for PECAM-1 in both the regulation of HSC migration to the chemokine CXCL12, as well as maintaining levels of the matrix degrading enzyme MMP-9 in the bone marrow vascular niche. Using intravital microscopy and adoptive transfer of either wild type (WT) or PECAM-1^−/− bone marrow precursors, we demonstrate that the increase in HSC progenitors in the blood is due in part to a reduced ability to migrate from blood to the bone marrow vascular niche. These findings suggest a novel role for PECAM-1 as a regulator of resting homeostatic progenitor cell numbers in the blood

Matrix metalloproteinases promote motor axon fasciculation in the Drosophila embryo

Matrix metalloproteinases (MMPs) are a large conserved family of extracellular proteases, a number of which are expressed during neuronal development and upregulated in nervous system diseases. Primarily on the basis of studies using pharmaceutical inhibitors, MMPs have been proposed to degrade the extracellular matrix to allow growth cone advance during development and hence play largely permissive roles in axon extension. Here we show that MMPs are not required for axon extension in the Drosophila embryo, but rather are specifically required for the execution of several stereotyped motor axon pathfinding decisions. The Drosophila genome contains only two MMP homologs, Mmp1 and Mmp2. We isolated Mmp1 in a misexpression screen to identify molecules required for motoneuron development. Misexpression of either MMP inhibits the regulated separation/defasciculation of motor axons at defined choice points. Conversely, motor nerves in Mmp1 and Mmp2 single mutants and Mmp1 Mmp2 double mutant embryos are loosely bundled/fasciculated, with ectopic axonal projections. Quantification of these phenotypes reveals that the genetic requirement for Mmp1 and Mmp2 is distinct in different nerve branches, although generally Mmp2 plays the predominant role in pathfinding. Using both an endogenous MMP inhibitor and MMP dominant-negative constructs, we demonstrate that MMP catalytic activity is required for motor axon fasciculation. In support of the model that MMPs promote fasciculation, we find that the defasciculation observed when MMP activity is compromised is suppressed by otherwise elevating interaxonal adhesion -- either by overexpressing Fas2 or by reducing Sema-1a dosage. These data demonstrate that MMP activity is essential for embryonic motor axon fasciculation.

Matrix metalloproteinases and their inhibitors in human traumatic spinal cord injury

BACKGROUND

Matrix metalloproteinases (MMPs) are a family of extracellular endopeptidases that degrade the extracellular matrix and other extracellular proteins. Studies in experimental animals demonstrate that MMPs play a number of roles in the detrimental as well as in the beneficial events after spinal cord injury (SCI). In the present correlative investigation, the expression pattern of several MMPs and their inhibitors has been investigated in the human spinal cord.

METHODS

An immunohistochemical investigation in post mortem samples of control and lesioned human spinal cords was performed. All patients with traumatic SCI had been clinically diagnosed as having "complete" injuries and presented lesions of the maceration type.

RESULTS

In the unlesioned human spinal cord, MMP and TIMP immunoreactivity was scarce. After traumatic SCI, a lesion-induced bi-phasic pattern of raised MMP-1 levels could be found with an early up-regulation in macrophages within the lesion epicentre and a later induction in peri-lesional activated astrocytes. There was an early and brief induction of MMP-2 at the lesion core in macrophages. MMP-9 and -12 expression peaked at 24 days after injury and both molecules were mostly expressed in macrophages at the lesion epicentre. Whereas MMP-9 levels rose progressively from 1 week to 3 weeks, there was an isolated peak of MMP-12 expression at 24 days. The post-traumatic distribution of the MMP inhibitors TIMP-1, -2 and -3 was limited. Only occasional TIMP immuno-positive macrophages could be detected at short survival times. The only clear induction was detected for TIMP-3 at survival times of 8 months and 1 year in peri-lesional activated astrocytes.

CONCLUSION

The involvement of MMP-1, -2, -9 and -12 has been demonstrated in the post-traumatic events after human SCI. With an expression pattern corresponding largely to prior experimental studies, they were mainly expressed during the first weeks after injury and were most likely involved in the destructive inflammatory events of protein breakdown and phagocytosis carried out by infiltrating neutrophils and macrophages, as well as being involved in enhanced permeability of the blood spinal cord barrier. Similar to animal investigations, the strong induction of MMPs was not accompanied by an expression of their inhibitors, allowing these proteins to exert their effects in the lesioned spinal cord.

Subventricular zone-derived neuroblast migration to the olfactory bulb is modulated by matrix remodelling

In the rodent brain neural progenitor cells are born in the subventricular zone and migrate along a pathway called the rostral migratory stream (RMS) into the olfactory bulb where they differentiate into several classes of interneurones. In the adult, tangential migration in the RMS takes place in 'chains' of cells contained within glial tubes. In contrast, neonatal neuroblasts along the RMS lack these defined glial tubes and chains, migrating instead as individual cells. Time-lapse confocal microscopy of neuroblasts at each of these ages shows that individual cells migrate in a saltatory manner with bursts of high speed followed by periods of slower speed. Tangential migration within a glial tube is 20% faster than migration as individual cells. Neuroblasts may also interact and modify the extracellular matrix during migration through expression of a family of proteins, the matrix metalloproteinases (MMPs). MMPs are present and active along the subventricular zone-olfactory bulb pathway. In the presence of inhibitors of MMPs, neuroblast migration rates were reduced only when cells migrate individually. Chain migration in the adult was unaffected by MMP inhibitors. Taken together, these data suggest that MMPs only influence migration as individual cells and not as chains.

CNS progenitor cells promote a permissive environment for neurite outgrowth via a matrix metalloproteinase-2-dependent mechanism

Transplantation of progenitor cells to the CNS has shown promise in neuronal and glial replacement and as a means of rescuing host neurons from apoptosis. Here we examined the effect of progenitor grafts on neurite extension in the degenerating retina of rd1 (retinal degeneration 1) mice. Transplantation of retinal progenitor cells induced increased matrix metalloproteinase-2 (MMP2) secretion, partly from activated glial cells, which was then activated by neuronally expressed MMP14. Active MMP2 resulted in proteolysis of the neurite outgrowth inhibitors CD44 and neurocan in the degenerative retina, allowing significantly increased neurite outgrowth across the border between abutting nondystrophic and rd1 retinas. Progenitor-induced enhancement of outgrowth was abrogated by an MMP inhibitor or by coculture with retinal explants from MMP2-/- mice. This study provides the first identification of an MMP2-dependent mechanism by which exogenous progenitor cells alter the host environment to promote neural regeneration. This suggests a novel therapeutic role for progenitor cells in the treatment of CNS degenerative diseases.

Regeneration of nigrostriatal dopaminergic axons by degradation of chondroitin sulfate is accompanied by elimination of the fibrotic scar and glia limitans in the lesion site

Chondroitin sulfate increases around a lesion site after central nervous system injury and is believed to be an impediment to axonal regeneration, because administration of chondroitinase ABC, a chondroitin sulfate-degrading enzyme, promotes axonal regeneration of central neurons. To examine the physiological role of chondroitin sulfate up-regulation after injury, the nigrostriatal dopaminergic axons were unilaterally transected in mice, and chondroitinase ABC was then injected into the lesion site. In mice transected only, tyrosine hydroxylase-immunoreactive axons did not extend across the lesion at 1 or 2 weeks after the transection. Immunoreactivities of chondroitin sulfate side chains and core protein of NG2 proteoglycan increased in and around the lesion site, and a fibrotic scar containing type IV collagen deposits developed in the lesion center. In contrast, in mice transected and treated with chondroitinase ABC, numerous tyrosine hydroxylase-immunoreactive axons were regenerated across the lesion at 1 and 2 weeks after the transection. In these animals, chondroitin sulfate immunoreactivity remarkably decreased, and immunoreactivity of 2B6 antibody, which recognizes the stub of degraded chondroitin sulfate side chains, was enhanced. Furthermore, the formation of a fibrotic scar and a glia limitans that surrounds the former was completely prevented, although type IV collagen immunoreactivity remained in newly formed blood capillaries around the lesion site. We discuss the question of whether the chondroitin sulfate is acting as a direct inhibitor of axonal regeneration or whether the observed changes are due to a prevention of the fibrotic scar formation and a rearrangement of astrocytic membranes.

Metalloproteinases: mediators of pathology and regeneration in the CNS

The matrix metalloproteinases and related A disintegrin and metalloproteinase enzymes are implicated in various diseases of the nervous system. However, metalloproteinases are increasingly being recognized as having beneficial roles during nervous system development and following injury. This review discusses general principles that govern the expression of metalloproteinases in the nervous system and their detrimental outcomes. It then focuses on the roles of metalloproteinases and their mechanisms in regulating neurogenesis, myelin formation and axonal growth. It is clear that metalloproteinases are important determinants in enabling recovery from injury to the nervous system.

ROCK inhibition with Y27632 activates astrocytes and increases their expression of neurite growth-inhibitory chondroitin sulfate proteoglycans

Inhibition of Rho-kinase (ROCK) with Y27632 stimulates sprouting by injured corticospinal tract and dorsal column tract axons, and accelerates functional recovery. However, regeneration of these axons across the glial scar was not observed. Here we examined the effects of Y27632 treatment on chondroitin sulfate proteoglycan (CSPG) expression by astrocytes, which are a key component of the reactive gliosis inhibiting axonal regeneration. In vivo, rats underwent a dorsal column transection and were treated with Y27632 via intrathecal pump infusion. Compared with controls, Y27632-treated injury sites displayed exaggerated upregulation of glial fibrillary acid protein and neurocan immunoreactivity along the lesion edge. In vitro, astrocytes assumed a reactive morphology (stellate shape) and increased their expression of CSPGs after Y27632 treatment. Neurite growth by dissociated cortical neurons decreased when cultured on the extracellular matrix (ECM) derived from Y27632-treated astrocytes. This decrease in neurite growth was reversed with chondroitinase-ABC (ChABC) digestion, indicating that the inhibition was due to CSPG depositions within the ECM. Interestingly, conditioned medium (CM) from untreated astrocytes was inhibitory to neurite growth, which was overcome by ChABC digestion. Such inhibitory activity was not found in the CM of Y27632-treated astrocytes. Taken together, these data support a model where ROCK inhibition by Y27632 modifies astrocytic processing of CSPGs, and increases the presence of CSPGs within the ECM while reduces CSPGs in the CM (cerebrospinal fluid in vivo). This increased expression of inhibitory CSPGs in the ECM of the glial scar may counteract the growth promoting effects of ROCK inhibition on axonal growth cones.

Matrix metalloproteinases and proteoglycans in axonal regeneration

After an injury to the adult mammalian central nervous system (CNS), a variety of growth-inhibitory molecules are upregulated. A glial scar forms at the site of injury and is composed of numerous molecular substances, including chondroitin sulfate proteoglycans (CSPGs). These proteoglycans inhibit axonal growth in vitro and in vivo. Matrix metalloproteinases (MMPs) can degrade the core protein of some CSPGs as well as other growth-inhibitory molecules such as Nogo and tenascin-C. MMPs have been shown to facilitate axonal regeneration in the adult mammalian peripheral nervous system (PNS). This review will focus on the various roles of proteoglycans and MMPs within the injured nervous system. First, we will present a general background on the injured central nervous system and explore the roles that proteoglycans play in the injured PNS and CNS. Second, we will discuss the various functions of MMPs within the injured PNS and CNS. Special attention will be paid to the possibility of how MMPs might modify the growth-inhibitory extracellular environment of the injured adult mammalian spinal cord and facilitate axonal regeneration in the CNS.

Modulation of extracellular matrix components by metalloproteinases and their tissue inhibitors during degeneration and regeneration of rat sural nerve

The success of peripheral nervous system regeneration has been associated with changes on the microenvironment, particularly on the extracellular matrix (ECM) components. In the present study we analyzed by indirect immunohistochemistry, electron microscopy and Western blotting, the distribution of ECM components, metalloproteinases (MMPs) and their tissue inhibitors (TIMPs), during Wallerian degeneration (WD) and nerve regeneration (2nd, 7th and 21st days after injury) on crushed rat sural nerves. Our results showed that laminin alpha3-chain and alpha2-chain are over expressed during the early stages of degeneration and regeneration respectively, whereas type IV collagen expression increased progressively after crush. On the other hand, the expression of chondroitin sulfate was down regulated during the early stages of degeneration, returning progressively to normal values during nerve regeneration. The expression of MMP-3 was almost normal immediately after lesion, and then reduced progressively achieving the smallest expression at 21 days after crush; on the contrary, the expression of MMP-7 increased significantly immediately after crush (2nd day) returning to normal values afterwards. TIMP-1 and TIMP-2 were over expressed at the beginning of WD, returning progressively to normal values after that. These results indicate that the modifications of ECM components, which are favorable for nerve regeneration, are correlated with changes on the balance between MMPs and TIMPs.

Matrix-mediated gene transfer to brain cortex and dorsal root ganglion neurones by retrograde axonal transport after dorsal column lesion

BACKGROUND

In previous studies, we showed that the immobilisation of DNAs encoding basic fibroblast growth factor, neurotrophin-3 and brain-derived neurotrophic factor in a gene-activated matrix (GAM) promotes sustained survival of axotomised retinal ganglion cells after optic nerve injury. Here, we evaluated if the immobilisation of DNAs in a GAM could be an effective approach to deliver genes to axotomised dorsal root ganglion (DRG) neurones after spinal cord injury and if the matrix component of the GAM would modulate the deposition of a dense scar at the injury site.

METHODS

We evaluated the expression of the thymidine kinase (TK) reporter gene in brain cortex and DRG after a bilateral T8 dorsal column (DC) lesion using PCR, RT-PCR and in situ hybridisation analyses. Collagen-based GAMs were implanted at the lesion site and the cellular response to the GAM was assessed using cell-specific markers.

RESULTS

At 1 week post-injury, PCR analyses confirmed that DNATK was retrogradely transported from the DC lesion where the GAM was implanted to the brain cortex and to caudal DRG neurones, and RT-PCR analyses showed expression of mRNATK. At 7 weeks post-injury, DNATK was still be detected in the GAM and DRG. In situ hybridisation localised DNATK and mRNATK within fibroblasts, glia, endothelial and inflammatory cells invading the GAM and in DRG neurones. Interestingly, the presence of a GAM also reduced secondary cavitation and scar deposition at the lesion site.

CONCLUSIONS

These results establish that GAMs act as bridging scaffolds in DC lesions limiting cavitation and scarring and delivering genes both locally to injury-reactive cells and distally to the cerebral cortex and to DRG neuronal somata through retrograde axonal transport.

TACE‐induced cleavage of NgR and p75 NTR in dorsal root ganglion cultures disinhibits outgrowth and promotes branching of neurites in the presence of inhibitory CNS myelin

After binding, central nervous system (CNS) myelin-derived axon growth inhibitory ligands, the Nogo-66 receptor (NgR), complexes with LINGO-1 and either the low-affinity neurotrophin receptor (p75(NTR)) or TROY to initiate growth cone collapse via a Rho-A inhibitory signaling pathway and/or Ca(2+)-dependent activation of epidermal growth factor receptor (EGFR) through an unknown signaling pathway. We have shown that axon growth through CNS myelin is disinhibited after neurotrophic factor administration by 1) initiating intramembranous proteolysis (RIP) of p75(NTR), leading to cleavage of the extracellular (p75(ECD)) and intracellular domains (p75(ICD)) by alpha- and gamma-secretase, respectively, thereby paralyzing inhibitory signaling; 2) shedding of soluble NgR(ECD), which acts as a competitive antagonist to NgR for binding of inhibitory ligands; and 3) antagonizing NgR/p75(NTR) clustering by competitive p75(ECD)/NgR interaction. Here, we report that TNF-alpha converting enzyme (TACE) (a disintegrin and metalloproteinase 17, ADAM17) induces disinhibition of FGF2-stimulated neurite outgrowth of dorsal root ganglion neurons (DRGN) cultured in the presence of a predetermined concentration of inhibitory CNS myelin-derived ligands. After addition of TACE (which has alpha-secretase activity) to mitotically arrested adult rat mixed DRG cultures, we demonstrate 1) NgR(ECD) shedding; 2) release of p75(ECD) and p75(ICD) by RIP of p75(NTR); 3) blockade of Rho-A activation; 4) reduced EGFR phosphorylation; and 5) increased FGF2-stimulated DRGN neurite outgrowth and branching in the presence of CNS myelin-derived inhibitory ligands. Thus, TACE-induced cleavage of NgR and RIP of p75(NTR) abrogates axon growth inhibitory signaling, thereby disinhibiting CNS axon/neurite growth.

Collagen matrix in spinal cord injury

The fibrous scar that develops after central nervous system (CNS) injury is considered a major impediment for axonal regeneration. It consists of a dense collagen IV meshwork, which serves as a binding matrix for numerous other extracellular matrix components and inhibitory molecules like proteoglycans and semaphorins, but also growth-promoting factors. Inhibition of collagen matrix formation in brain and spinal cord lesions leads to axonal regeneration and functional recovery, although collagen IV per se is not inhibitory for axonal outgrowth. This review focuses on the molecular properties of the collagen IV matrix and its interactions with various molecules that are expressed after CNS lesion. Moreover, studies on collagen expression and matrix formation after injury of regenerating versus non-regenerating nervous systems are reviewed. Major differences in collagen deposition in the CNS and the peripheral nervous system (PNS) and differences in specific cell responses to extracellular matrix deposition in the lesion area are discussed. Therapeutic treatments aiming at suppression of fibrous scarring have been shown to promote axon regeneration in various lesion paradigms of the mammalian CNS.

Degenerative and spontaneous regenerative processes after spinal cord injury

Spinal cord injury results in acute as well as progressive secondary destruction of local and distant nervous tissue through a number of degenerative mechanisms. Spinal cord injury also initiates a number of endogenous neuroprotective and regenerative responses. Understanding of these mechanisms might identify potential targets for treatments after spinal cord injury in humans. Here, we first discuss recent developments in our understanding of the immediate traumatic and subsequent secondary degeneration of local tissue and long projecting pathways in animal models. These include the inflammatory and vascular responses during the acute phase, as well as cell death, demyelination and scar formation in the subacute and chronic phases. Secondly, we discuss the spontaneous axonal regeneration of injured and plasticity of uninjured systems, and other repair-related responses in animals, including the upregulation of regeneration-associated genes in some neurons, increases in neurotrophic factors in the spinal cord and remyelination by oligodendrocyte precursors and invading Schwann cells. Lastly, we comment on the still limited understanding of the neuropathology in humans, which is largely similar to that in rodents. However, there also are potentially important differences, including the reduced glial scarring, inflammation and demyelination, the increased Schwannosis and the protracted Wallerian degeneration in humans. The validity of current rodent models for human spinal cord injury is also discussed. The emphasis of this review is on the literature from 2002 to early 2005.

Genes associated with adult axon regeneration promoted by olfactory ensheathing cells: a new role for matrix metalloproteinase 2

The molecular mechanisms used by olfactory ensheathing cells (OECs) to promote repair in the damaged adult mammalian CNS remain unknown. Thus, we used microarrays to analyze three OEC populations with different capacities to promote axonal regeneration in cultured rat retinal neurons. Gene expression in "long-term cultured OECs" that do not stimulate adult axonal outgrowth was compared with that of "primary olfactory ensheathing cells" and the immortalized OEC cell line TEG3. In this way, we identified a number of candidate genes that might play a role in promoting adult axonal regeneration. Among these genes, it was striking that both the matrix metalloproteinase 2 (MMP2) and an inhibitor of this protease were represented. The disruption of MMP2 activity in TEG3 cells impaired their capacity to trigger axon regeneration in cultured adult retinal neurons. Furthermore, the MMP2 protein was detected in grafts of OECs that elicited robust axonal regeneration in the injured spinal cord of adult rats in vivo. These data suggest that MMP2 does indeed participate in adult axonal regeneration induced by OECs.

Sensory neurite outgrowth on white matter astrocytes is influenced by intracellular and extracellular S100A4 protein

The central nervous system (CNS) is considered a nonpermissive environment for axonal regeneration because of the presence of myelin and associated repulsive molecules. However, neural cells transplanted to the CNS preferably migrate and extend their fibers in white matter areas. We previously showed that white matter astrocytes in vivo express the calcium-binding protein S100A4, which is strongly up-regulated in areas of white matter degeneration. To investigate the role of white matter astrocytes and their specific protein S100A4 in axonal regeneration, we developed white matter astrocyte cultures with strong S100A4 expression and grew dissociated adult dorsal root ganglion (DRG) cells on top of astrocytes for 24 hr. By using small interfering S100A4 RNA, we were able to eliminate S100A4 expression and compare growth of DRG cell neurites on S100A4-silenced and S100A4-expressing astrocytes. In addition, we studied whether extracellular S100A4 has an effect on neurite growth from adult DRG cells cultured on S100A4-expressing white matter astrocytes. Our data show that white matter astrocytes are permissive for neurite growth, although high levels of S100A4 in white matter astrocytes have a negative effect on this growth. Extracellular application of S100A4 induced extensive growth of DRG cell neurites on white matter astrocytes. These findings suggest that white matter astrocytes are able to support axonal regeneration and, furthermore, that administration of extracellular S100A4 provides strong additional support for axonal regeneration.

Distribution and expression of tissue inhibitors of metalloproteinase in dorsal root entry zone and dorsal column after dorsal root injury

To understand whether tissue inhibitors of metalloproteinase (TIMPs) contribute to the failure of regenerating sensory axons to enter the spinal cord, we used in situ hybridization and immunocytochemistry to examine the expression of TIMP1, TIMP2, and TIMP3 in the dorsal root, dorsal root entry zone (DREZ), and dorsal column after dorsal root injury in adult rats. We found that the three TIMPs and their mRNAs were up-regulated in a time-, region-, and cell-type-specific manner. Strong up-regulation of all three TIMPs was seen in the injured dorsal roots. TIMP2 was also significantly up-regulated in the DREZ and degenerating dorsal column, where TIMP1 and TIMP3 showed only moderate up-regulation. Most cells up-regulating the TIMPs in the DREZ and degenerating dorsal column were reactive astrocytes, but TIMP2 was also up-regulated by microglia/macrophages, especially at long postoperative survival times. These results suggest that TIMPs may be involved in controlling tissue remodelling following dorsal root injury and that manipulation of the expression of TIMPs may provide a means of promoting axonal regeneration into and within the injured spinal cord.

Neurotrophic factor synergy is required for neuronal survival and disinhibited axon regeneration after CNS injury

The therapeutic effects of individual neurotrophic factors (NTF) have proved disappointing in clinical trials for neuronal repair and axon regeneration. Here, we demonstrate NTF synergistic neuronal responses after a combination of basic fibroblast growth factor, neurotrophin-3 and brain derived growth factor delivered to the somata of retinal ganglion cells promoted greater survival and axon growth than did the sum of the effects of each NTF alone. Triple and not single NTF treatments potentiated regulated intramembraneous proteolysis of p75(NTR), and ectodomain shedding of Nogo receptor, correlated with a 30% decrease in activation of Rho-A, a key signalling molecule in the axon growth inhibitory cascade. Thus, combinatorial NTF administration synergistically enhanced neuronal survival, disinhibited axon growth and promoted axon regeneration through the hostile CNS environment without the intervention of scar tissue at the lesion site.

Aryl hydrocarbon receptor activation inhibits regenerative growth

There is considerable literature supporting the conclusion that inappropriate activation of the aryl hydrocarbon receptor (AHR) alters cellular signaling. We have established previously that fin regeneration is specifically inhibited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in adult zebrafish and have used this in vivo endpoint to evaluate interactions between AHR and growth-controlling pathways. Because there are experimental limitations in studying regeneration in adult animals, we have developed a larval model to evaluate the effect of AHR activation on tissue regeneration. Two-day-old zebrafish regenerate their amputated caudal fins within 3 days. Here, we demonstrate that TCDD specifically blocks regenerative growth in larvae. The AHR pathway in zebrafish is considerably more complex than in mammals, with at least three zebrafish AHR genes (zfAHR1a, zfAHR1b, and zfAHR2) and two ARNT genes (zfARNT1 and zfARNT2). Although it was presumed that the block in regeneration was mediated by AHR activation, it had not been experimentally demonstrated. Using antisense morpholinos and mutant fish lines, we report that zfAHR2 and zfARNT1 are the in vivo dimerization partners that are required for inhibition of regeneration by TCDD. Several pathways including fibroblast growth factor (FGF) signaling are essential for fin regeneration. Even though impaired FGF signaling and TCDD exposure both inhibit fin regeneration, their morphometric response is distinct, suggesting that the mechanisms of impairment are different. With the plethora of molecular and genetic techniques that can be applied to larval-stage embryos, this in vivo regeneration system can be further exploited to understand cross-talk between AHR and other signaling pathways.