Spinal axon regeneration evoked by replacing two growth cone proteins in adult neurons

Spinal axon regeneration induced by elevation of cyclic AMP

Myelin inhibitors, including MAG, are major impediments to CNS regeneration. However, CNS axons of DRGs regenerate if the peripheral branch of these neurons is lesioned first. We show that 1 day post-peripheral-lesion, DRG-cAMP levels triple and MAG/myelin no longer inhibit growth, an effect that is PKA dependent. By 1 week post-lesion, DRG-cAMP returns to control, but growth on MAG/myelin improves and is now PKA independent. Inhibiting PKA in vivo blocks the post-lesion growth on MAG/myelin at 1 day and attenuates it at 1 week. Alone, injection of db-cAMP into the DRG mimics completely a conditioning lesion as DRGs grow on MAG/myelin, initially, in a PKA-dependent manner that becomes PKA independent. Importantly, DRG injection of db-cAMP results in extensive regeneration of dorsal column axons lesioned 1 week later. These results may be relevant to developing therapies for spinal cord injury.

Involvement of Cajal-Retzius cells in robust and layer-specific regeneration of the entorhino-hippocampal pathways

Severed adult CNS axons can extend over long distances when a permissive 'milieu', such as grafted Schwann cells or ensheathing cells, is provided. Moreover, functional blocking of endogenous inhibitory factors, such as Nogo-A or proteoglycans, enhances the regeneration of axotomized neurons. Here we examine whether guidance cues available during the development of axonal pathways could also potentiate the regeneration of lesioned adult circuits. The Cajal-Retzius cells in the hippocampus are transient pioneer neurons that guide entorhino-hippocampal afferents to their target layers. By using an in vitro model of axotomy of the entorhino-hippocampal pathway we show that Cajal-Retzius cells triggered the regeneration of the axotomized entorhino-hippocampal pathway. Furthermore, the regrowth induced by Cajal-Retzius cells was robust and its pattern was indistinguishable from that of the unlesioned entorhino-hippocampal pathway. Thus, regenerating axons regrew in a layer-specific fashion towards the appropriate target layers, making synaptic contacts with target pyramidal neurons. Interestingly, the ability of lesioned entorhinal axons to regrow was maintained for at least 9 days after axotomy. These results show that the growth-promoting cells controlling the development of neural circuits will be a relevant approach to promoting the regeneration of lesioned adult CNS pathways.

Change of gene expression profiles in the retina following optic nerve injury

The purpose of the present study was to search for changes in gene expression patterns in the retina following optic nerve injury. We conducted a subtractive hybridization for comparison of the mRNAs in those retinas receiving optic nerve crush injury and those receiving sham operation. Both forward and reverse subtractions were carried out for 8-h and 24-h time points postoperatively. Resultant subtractive cDNA for each group was re-amplified and cloned to a plasmid. After DNA sequencing, the identity of subtractive cDNA was analyzed by blasting sequences to the Nonredundant gene database, Unigene database, and dbest database at NCBI. Thirty-four known genes and 32 EST were found in the forward subtractions. Forty-two known genes and 46 EST were found in reverse subtractions. Identities of the rest could not been found in the databases. To verify the subtractive results, RT-PCR was performed to test expression patterns of eight known genes found in the above analysis. Among these eight genes, seven demonstrated a statistically significant difference between the crushed eyes and the control eyes by quantitative image analysis. Together, our data show that expression of fatso, ephrin B2, NonO, Zfx, vitronectin, and XLRS increased after optic nerve injury, and expression of stathmin exhibited reduction after injury.

Up-regulation of cytochrome oxidase in the retina following optic nerve injury

The purpose of this study is to investigate the cytochrome oxidase (COX) activity in the retina and optic nerve following an optic nerve injury. The optic nerve crush of one eye was carried out in Balb/c mice. A semi-quantitative RT-PCR method was then adopted to evaluate the mRNA expression of cytochrome oxidase subunit 1 (COX1) in the retina after surgery. Up-regulation of COX1 mRNA in the retina was detected by RT-PCR at 24 hr following the optic nerve injury. Total retinal mitochondrial mass measured by fluorescent intensity of MitoTracker green was not altered following the injury. COX histochemistry performed on cryostat sections showed an elevated enzyme activity of COX in the retina and in the optic nerve. In the retina, elevation of the COX activity was observed in the retinal ganglion cell layer and the overlying nerve fibre layer. The increase of COX activity began from 24 hr after injury, peaked around day 3, and maintained up to 1 week after the operation. In the optic nerve, increase of COX activity was observed in regions distal to the crush line and distributed either randomly or in a cone shape. In conclusion, both the expression of COX1 mRNA in retina and the activity of COX in inner plexiform layer and retinal ganglion cell layer were elevated following optic nerve injury without affecting total retinal mitochondrial mass. These findings suggested that one of early responses in the retina and in the optic nerve after the optic nerve injury is to scale up the energy production.

Promoting axonal regeneration in the central nervous system by enhancing the cell body response to axotomy

Neurons projecting into the peripheral nervous system (PNS) regenerate their axons after injury, in contrast to those confined to the central nervous system (CNS). Both neuronal and nonneuronal factors contribute to the lack of CNS regeneration. In this review we concentrate on the differential gene expression response to axotomy in PNS vs. CNS neurons. In general CNS neurons fail to up-regulate or sustain the expression of regeneration-associated proteins (RAGs), including trophic factors and their receptors. The presumed lack of trophic support of axotomized CNS neurons provided the rationale for the exogenous application of trophic factors, either to the lesion site or to the cell bodies. Here, we review our data on the application of trophic factors to rubrospinal and corticospinal neurons. Cell body treatment of axotomized rubrospinal neurons with brain-derived neurotrophic factor (BDNF) reversed atrophy, increased GAP-43 and Talpha-1 tubulin mRNA expression, and promoted axonal regeneration into peripheral nerve grafts. Importantly, BDNF cell body treatment was still effective in the chronic setting, i.e., when initiated 1 year after injury, but BDNF had no effect when applied to the chronic spinal cord injury site. The ability to promote regeneration in chronically injured neurons will hopefully contribute to the development of treatment strategies for chronic spinal injuries.

Survival and regeneration of rubrospinal neurons 1 year after spinal cord injury

Scientific interest to find a treatment for spinal cord injuries has led to the development of numerous experimental strategies to promote axonal regeneration across the spinal cord injury site. Although these strategies have been developed in acute injury paradigms and hold promise for individuals with spinal cord injuries in the future, little is known about their applicability for the vast majority of paralyzed individuals whose injury occurred long ago and who are considered to have a chronic injury. Some studies have shown that the effectiveness of these approaches diminishes dramatically within weeks after injury. Here we investigated the regenerative capacity of rat rubrospinal neurons whose axons were cut in the cervical spinal cord 1 year before. Contrary to earlier reports, we found that rubrospinal neurons do not die after axotomy but, rather, they undergo massive atrophy that can be reversed by applying brain-derived neurotrophic factor to the cell bodies in the midbrain. This administration of neurotrophic factor to the cell body resulted in increased expression of growth-associated protein-43 and Talpha1 tubulin, genes thought to be related to axonal regeneration. This treatment promoted the regeneration of these chronically injured rubrospinal axons into peripheral nerve transplants engrafted at the spinal cord injury site. This outcome is a demonstration of the regenerative capacity of spinal cord projection neurons a full year after axotomy.

Expression of activating transcription factor 3 and growth-associated protein 43 in the rat geniculate ganglion neurons after chorda tympani injury

The purpose of this study was to evaluate the degree of damage in the geniculate ganglion and its target organ as a result of chorda tympani (CT) injury. We performed unilateral transection of the rat CT and examined expression of the activating transcription factor 3 (ATF3), a neuronal injury marker, and the growth-associated protein 43 (GAP-43), a regeneration-associated molecule. The mean proportion of ATF3-immunoreactive (ir) neurons in the geniculate ganglion was approximately 32% at 3 days after CT injury, but these neurons were never detected in the naive ganglion. Using in situ hybridization, the mean percentage of GAP-43 mRNA-labeled neurons (signal : noise ratio > or = 10) was observed to have increased significantly to approximately 60% for 1-7 days after CT injury, while that in the naive ganglion was < 15%. The results of morphological studies using scanning electron microscopy and immunohistochemistry indicated that atrophic change and reduction of protein gene-product 9.5-ir fibers in the denervated papillae, mainly in the intragemmal region, were observed after CT injury. Increase in GAP-43 mRNA, suggesting CT axonal regeneration, may have a role in recovery from taste disorders. However, this regenerative process may be involved in abnormal activity in the axotomized neurons or the adjacent intact neurons and so one must not disregard the existence of injured geniculate ganglions when considering the treatment of diseases that cause CT injury.

Spinal cord repair in neonatal rats: a correlation between axonal regeneration and functional recovery

The present study aimed to analyse how anatomical regeneration contributes to functional recovery after experimental spinal cord repair. Thoracic spinal cord of neonatal rats was completely transected to make a gap and repaired by grafting a section of embryonic spinal cord. Six weeks after surgery, outcome of locomotor performance was assessed using an open field locomotor scale (BBB scale). Axonal regeneration across the repaired site was quantitatively assessed in the raphe, vestibular, and red nuclei and the sensorimotor cortex by a retrograde tracing method. The rats that had no labelled neurons in any of the supraspinal nuclei showed no hind-forelimb coordination. The rats that had labelled neurons in the brainstem nuclei but not in the sensorimotor cortex showed hind-forelimb coordination of varying grades depending on the amount of regeneration. The rats that had labelled neurons in all of the examined nuclei showed almost normal locomotion. In addition to a relationship between distribution of the labelled neurons and functional recovery, a positive correlation was observed between number of the labelled neurons in each of the supraspinal nuclei and locomotor performance of the rat. Thus the grade of restored function appeared to be regulated by distribution and number of fibres regenerated across the repaired site and into the target region. These results suggest that accurate reconstruction of neural connections is essential for significant functional recovery after spinal cord repair.

Nogos and the Nogo-66 receptor: factors inhibiting CNS neuron regeneration

The recently cloned gene Nogo, whose alternative splice products correspond to the antigenic target of the central nervous system (CNS) regeneration enhancing monoclonal antibody IN-1, codes for membrane proteins enriched in brain, particularly in oligodendrocytes. The 66-amino acid extracellular domain of Nogo (Nogo-66) interacts with a high-affinity receptor (NgR), a glycosylphosphatidylinositol (GPI)-linked protein with multiple leucine-rich repeats. The amino terminal cytoplasmic domain of Nogo appears to have a general cellular growth inhibitory effect. Nogo-66, on the other hand, specifically retards neurite outgrowth and induces growth cone collapse, possibly through its interaction with NgR and as yet unidentified transmembrane coreceptors. Recent results also suggest that Nogo expression may induce apoptosis in tumor cells. Together, these proteins provide new molecular handles for the design of therapeutic interventions for CNS injuries and neurodegenerative diseases, as well as possible leads to anticancer strategies.

Small proline-rich repeat protein 1A is expressed by axotomized neurons and promotes axonal outgrowth

The ability of neurons to regenerate an axon after injury is determined by both the surrounding environment and factors intrinsic to the damaged neuron. We have used cDNA microarrays to survey those genes induced during successful sciatic nerve regeneration. The small proline-rich repeat protein 1A (SPRR1A) is not detectable in uninjured neurons but is induced by >60-fold after peripheral axonal damage. The protein is localized to injured neurons and axons. sprr1a is one of a group of epithelial differentiation genes, including s100c and p21/waf, that are coinduced in neurons by axotomy. Overexpressed SPRR1A colocalizes with F-actin in membrane ruffles and augments axonal outgrowth on a range of substrates. In axotomized sensory neurons, reduction of SPRR1A function restricts axonal outgrowth. Neuronal SPRR1A may be a significant contributor to successful nerve regeneration.

Small proline-rich repeat protein 1A is expressed by axotomized neurons and promotes axonal outgrowth

The ability of neurons to regenerate an axon after injury is determined by both the surrounding environment and factors intrinsic to the damaged neuron. We have used cDNA microarrays to survey those genes induced during successful sciatic nerve regeneration. The small proline-rich repeat protein 1A (SPRR1A) is not detectable in uninjured neurons but is induced by >60-fold after peripheral axonal damage. The protein is localized to injured neurons and axons. sprr1a is one of a group of epithelial differentiation genes, including s100c and p21/waf, that are coinduced in neurons by axotomy. Overexpressed SPRR1A colocalizes with F-actin in membrane ruffles and augments axonal outgrowth on a range of substrates. In axotomized sensory neurons, reduction of SPRR1A function restricts axonal outgrowth. Neuronal SPRR1A may be a significant contributor to successful nerve regeneration.

Constitutive overexpression of the basic helix-loop-helix Nex1/MATH-2 transcription factor promotes neuronal differentiation of PC12 cells and neurite regeneration

Elucidation of the intricate transcriptional pathways leading to neural differentiation and the establishment of neuronal identity is critical to the understanding and design of therapeutic approaches. Among the important players, the basic helix-loop-helix (bHLH) transcription factors have been found to be pivotal regulators of neurogenesis. In this study, we investigate the role of the bHLH differentiation factor Nex1/MATH-2 in conjunction with the nerve growth factor (NGF) signaling pathway using the rat phenochromocytoma PC12 cell line. We report that the expression of Nex1 protein is induced after 5 hr of NGF treatment and reaches maximal levels at 24 hr, when very few PC12 cells have begun extending neurites and ceased cell division. Furthermore, our study demonstrates that Nex1 has the ability to trigger neuronal differentiation of PC12 cells in the absence of neurotrophic factor. We show that Nex1 plays an important role in neurite outgrowth and has the capacity to regenerate neurite outgrowth in the absence of NGF. These results are corroborated by the fact that Nex1 targets a repertoire of distinct types of genes associated with neuronal differentiation, such as GAP-43, betaIII-tubulin, and NeuroD. In addition, our findings show that Nex1 up-regulates the expression of the mitotic inhibitor p21(WAF1), thus linking neuronal differentiation to cell cycle withdrawal. Finally, our studies show that overexpression of a Nex1 mutant has the ability to block the execution of NGF-induced differentiation program, suggesting that Nex1 may be an important effector of the NGF signaling pathway.

Olfactory Ensheathing Glia: Drivers of Axonal Regeneration in the Central Nervous System?

Olfactory ensheathing glia (OEG) accompany olfactory growing axons in their entry to the adult mammalian central nervous system (CNS). Due to this special characteristic, considerable attention has been focused on the possibility of using OEG for CNS regeneration. OEG present a large heterogeneity in culture with respect to their cellular morphology and expressed molecules. The specific characteristics of OEG responsible for their regenerative properties have to be defined. These properties probably result from the combination of several factors: molecular composition of the membrane (expressing adhesion molecules as PSA-NCAM, L1 and/or others) combined with their ability to reduce glial scarring and to accompany new growing axons into the host CNS. Their capacity to produce some neurotrophic factors might also account for their ability to produce CNS regeneration.

Spinal cord regeneration: from gene to transplants

The past 20 years has seen the emergence of many exciting and promising experimental therapeutic strategies to promote regeneration of the injured spinal cord in laboratory animals. A greater understanding of the pathophysiologic mechanisms that contribute to the initial and secondary cord injury may facilitate the development of neuroprotective strategies that preserve axonal function and prevent apoptotic cell death, thus optimizing neurologic function. Neurotrophic factors have been used to augment the poor intrinsic regenerative capacity of central nervous system neurons, and the need for sophisticated delivery of such trophic agents has stimulated the application of gene therapy in this context. In addition to augmenting the neuronal capacity to regenerate axons, many researchers are developing strategies to overcome the inhibitory environment into which these axons must grow. Characterizing the inhibitory elements of the glial scar at the site of injury and of myelin in the distal tracts is therefore a focus of intense scientific interest. To this effect, a number of strategies have also been developed to bridge the injury site and facilitate axonal growth across the lesion with a variety of cellular substrates. These include fetal tissue transplants, stem cells, Schwann cells, and olfactory ensheathing cells. With the collaboration of basic scientists and clinicians, it is hoped that these experimental strategies coupled with a greater understanding of the neurobiology of spinal cord injury will be translatable to the clinical setting in the near future.

Nerve growth factor- and neurotrophin-3-releasing guidance channels promote regeneration of the transected rat dorsal root

Dorsal roots have a limited regeneration capacity after transection. To improve nerve regeneration, the growth-promoting effects of the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) were evaluated. The proteins were continuously released by synthetic nerve guidance channels bridging a 4-mm gap in the transected dorsal root. Four weeks after lesion, the regenerated nerve cables were analyzed for the presence of myelinated and unmyelinated axons. While BDNF showed a limited effect on axonal regeneration (863 +/- 39 axons/regenerated nerve, n = 6), NGF (1843 +/- 482) and NT-3 (1495 +/- 449) powerfully promoted regeneration of myelinated axons compared to channels releasing the control protein bovine serum albumin (293 +/- 39). In addition, NGF, but not BDNF nor NT-3, had a potent effect on the regeneration of unmyelinated axons (NGF, 55 +/- 1.4; BDNF, 4 +/- 0.3; NT-3, 4.7 +/- 0.3 axons/100 microm(2); n = 6). The present study suggests that synthetic nerve guidance channels slowly and continuously releasing the neurotrophins NGF and NT-3 can overcome the limited regeneration of transected dorsal root.

Leukemia inhibitory factor determines the growth status of injured adult sensory neurons

Conditioning injury to adult mammalian sensory neurons enhances their regeneration potential. Here we show that leukemia inhibitory factor (LIF) is a fundamental component of the conditioning response. Conditioning injury in vivo significantly increases the intrinsic growth capacity of sensory neurons in vitro from LIF+/+ mice. This conditioning effect is significantly blunted in sensory neurons from LIF-/- mice. Enhanced growth is rescued in vitro in LIF-/- mice by the addition of exogenous LIF, and the effect blocked by human LIF-05, an LIF receptor antagonist. Furthermore, we demonstrate that LIF promotes elongating but not arborizing neurite outgrowth in vitro and is required for normal regeneration of injured adult sensory neurons in vivo. LIF is also functionally protective to peptidergic sensory neurons after nerve damage in vivo. Our results indicate that the alteration in intrinsic growth status of injured sensory neurons depends, at least in part, on LIF.

Protein-induced formation of cholesterol-rich domains

A major protein of neuronal rafts, NAP-22, binds specifically to cholesterol. We demonstrate by circular dichroism that NAP-22 contains a significant amount of beta-structure that is not sensitive to binding of the protein to membranes, suggesting that a major portion of the protein does not insert deeply into the membrane. The free energy of binding of NAP-22 to liposomes of dioleoylphosphatidylcholine with 40% cholesterol is -7.3 +/- 0.5 kcal/mol. NAP-22 mixed with dipalmitoylphosphatidylcholine and 40% cholesterol partitions into the detergent insoluble fraction in the presence of 1% Triton X-100. NAP-22 also causes this insoluble fraction to become enriched in cholesterol relative to phospholipid, again demonstrating the ability of this protein to segregate cholesterol and phospholipids into domains. Differential scanning calorimetry results demonstrate that NAP-22 promotes domain formation in liposomes composed of cholesterol and phosphatidylcholine. This is shown by NAP-22-promoted changes in the shape and enthalpy of the phase transition of phosphatidylcholine as well as by the appearance of cholesterol crystallite transitions in membranes composed of phosphatidylcholine with either saturated or unsaturated acyl chains. In situ atomic force microscopy revealed a marked change in the surface morphology of a supported bilayer of dioleoylphosphatidylcholine with 0.4 mole fraction of cholesterol upon addition of NAP-22. Prior to the addition of the protein, the bilayer appears to be a molecularly smooth structure with uniform thickness. Addition of NAP-22 resulted in the rapid formation of localized raised bilayer domains. Remarkably, there was no gross disruption or erosion of the bilayer but rather simply an apparent rearrangement of the lipid bilayer structure due to the interaction of NAP-22 with the lipid. Our results demonstrate that NAP-22 can induce the formation of cholesterol-rich domains in membranes. This is likely to be relevant in neuronal membrane domains that are rich in NAP-22.

New EMBO members' review: actin cytoskeleton regulation through modulation of PI(4,5)P(2) rafts

The phosphoinositide lipid PI(4,5)P(2) is now established as a key cofactor in signaling to the actin cytoskeleton and in vesicle trafficking. PI(4,5)P(2) accumulates at membrane rafts and promotes local co-recruitment and activation of specific signaling components at the cell membrane. PI(4,5)P(2) rafts may thus be platforms for local regulation of morphogenetic activity at the cell membrane. Raft PI(4,5)P(2) is regulated by lipid kinases (PI5-kinases) and lipid phosphatases (e.g. synaptojanin). In addition, GAP43-like proteins have recently emerged as a group of PI(4,5)P(2) raft-modulating proteins. These locally abundant proteins accumulate at inner leaflet plasmalemmal rafts where they bind to and co-distribute with PI(4,5)P(2), and promote actin cytoskeleton accumulation and dynamics. In keeping with their proposed role as positive modulators of PI(4,5)P(2) raft function, GAP43-like proteins confer competence for regulated morphogenetic activity on cells that express them. Their function has been investigated extensively in the nervous system, where their expression promotes neurite outgrowth, anatomical plasticity and nerve regeneration. Extrinsic signals and intrinsic factors may thus converge to modulate PI(4,5)P(2) rafts, upstream of regulated activity at the cell surface.

Graded expression patterns of ephrin-As in the superior colliculus after lesion of the adult mouse optic nerve

The idea has been put forward that molecules and mechanisms acting during development are re-used during regeneration in the adult, for example in response to traumatic injury in order to re-establish the functional integrity of neuronal circuits. Members of the Eph family of receptor tyrosine kinases and their 'ligands', the ephrins, play a prominent role during development of the retinocollicular projection in rodents, where EphA receptors and ephrin-As are expressed in gradients in both the retina and the superior colliculi (SC). We were interested in investigating whether EphA family members are also expressed or re-expressed in the adult after optic nerve lesion, since the presence of axon guidance information is an important prerequisite for a topographically appropriate re-connection by retinal ganglion cell (RGC) axons. This analysis was encouraged by results showing that RGC axons do not exert guidance preferences in response to membranes from adult unlesioned SC, but in response to membranes from the adult deafferented SC. We found a graded expression pattern of ephrin-As in the SC both before and after deafferentation, which was remarkably similar to those found during development. EphA receptor levels were reduced in the SC after deafferentation and the expression patterns of the EphB family were not changed. In particular, the presence of a graded ephrin-A expression in the deafferented SC suggests that - if robust regeneration of RGC axons can be achieved - topographic guidance information as a likely requirement for a functionally successful re-establishment of the retinocollicular projection is available.

Adult neuronal regeneration induced by transgenic integrin expression

In a variety of adult CNS injury models, embryonic neurons exhibit superior regenerative performance when compared with adult neurons. It is unknown how young neurons extend axons in the injured adult brain, in which adult neurons fail to regenerate. This study shows that cultured adult neurons do not adapt to conditions that are characteristic of the injured adult CNS: low levels of growth-promoting molecules and the presence of inhibitory proteoglycans. In contrast, young neurons readily adapt to these same conditions, and adaptation is accompanied by an increase in the expression of receptors for growth-promoting molecules (receptors of the integrin family). Surprisingly, the regenerative performance of adult neurons can be restored to that of young neurons by gene transfer-mediated expression of a single alpha-integrin.

Olfactory ensheathing cells - another miracle cure for spinal cord injury?

Several recent publications describe remarkably promising effects of transplanting olfactory ensheathing cells as a potential future method to repair human spinal cord injuries. But why were cells from the nose transplanted into the spinal cord? What are olfactory ensheathing cells, and how might they produce these beneficial effects? And more generally, what do we mean by spinal cord injury? To what extent can we compare repair in an animal to repair in a human?