GAP-43 in the axons of mammalian CNS neurons regenerating into peripheral nerve grafts

Axonal Growth Arrests After an Increased Accumulation of Schwann Cells Expressing Senescence Markers and Stromal Cells in Acellular Nerve Allografts

Acellular nerve allografts (ANAs) and other nerve constructs do not reliably facilitate axonal regeneration across long defects (>3 cm). Causes for this deficiency are poorly understood. In this study, we determined what cells are present within ANAs before axonal growth arrest in nerve constructs and if these cells express markers of cellular stress and senescence. Using the Thy1-GFP rat and serial imaging, we identified the time and location of axonal growth arrest in long (6 cm) ANAs. Axonal growth halted within long ANAs by 4 weeks, while axons successfully regenerated across short (3 cm) ANAs. Cellular populations and markers of senescence were determined using immunohistochemistry, histology, and senescence-associated β-galactosidase staining. Both short and long ANAs were robustly repopulated with Schwann cells (SCs) and stromal cells by 2 weeks. Schwann cells (S100β(+)) represented the majority of cells repopulating both ANAs. Overall, both ANAs demonstrated similar cellular populations with the exception of increased stromal cells (fibronectin(+)/S100β(-)/CD68(-) cells) in long ANAs. Characterization of ANAs for markers of cellular senescence revealed that long ANAs accumulated much greater levels of senescence markers and a greater percentage of Schwann cells expressing the senescence marker p16 compared to short ANAs. To establish the impact of the long ANA environment on axonal regeneration, short ANAs (2 cm) that would normally support axonal regeneration were generated from long ANAs near the time of axonal growth arrest ("stressed" ANAs). These stressed ANAs contained mainly S100β(+)/p16(+) cells and markedly reduced axonal regeneration. In additional experiments, removal of the distal portion (4 cm) of long ANAs near the time of axonal growth arrest and replacement with long isografts (4 cm) rescued axonal regeneration across the defect. Neuronal culture derived from nerve following axonal growth arrest in long ANAs revealed no deficits in axonal extension. Overall, this evidence demonstrates that long ANAs are repopulated with increased p16(+) Schwann cells and stromal cells compared to short ANAs, suggesting a role for these cells in poor axonal regeneration across nerve constructs.

Electrical Stimulation to Enhance Axon Regeneration After Peripheral Nerve Injuries in Animal Models and Humans

Injured peripheral nerves regenerate their lost axons but functional recovery in humans is frequently disappointing. This is so particularly when injuries require regeneration over long distances and/or over long time periods. Fat replacement of chronically denervated muscles, a commonly accepted explanation, does not account for poor functional recovery. Rather, the basis for the poor nerve regeneration is the transient expression of growth-associated genes that accounts for declining regenerative capacity of neurons and the regenerative support of Schwann cells over time. Brief low-frequency electrical stimulation accelerates motor and sensory axon outgrowth across injury sites that, even after delayed surgical repair of injured nerves in animal models and patients, enhances nerve regeneration and target reinnervation. The stimulation elevates neuronal cyclic adenosine monophosphate and, in turn, the expression of neurotrophic factors and other growth-associated genes, including cytoskeletal proteins. Electrical stimulation of denervated muscles immediately after nerve transection and surgical repair also accelerates muscle reinnervation but, at this time, how the daily requirement of long-duration electrical pulses can be delivered to muscles remains a practical issue prior to translation to patients. Finally, the technique of inserting autologous nerve grafts that bridge between a donor nerve and an adjacent recipient denervated nerve stump significantly improves nerve regeneration after delayed nerve repair, the donor nerves sustaining the capacity of the denervated Schwann cells to support nerve regeneration. These reviewed methods to promote nerve regeneration and, in turn, to enhance functional recovery after nerve injury and surgical repair are sufficiently promising for early translation to the clinic.

hnRNP-Q1 represses nascent axon growth in cortical neurons by inhibiting Gap-43 mRNA translation

A novel posttranscriptional mechanism for regulating the neuronal protein GAP-43 is reported. The mRNA-binding protein hnRNP-Q1 represses Gap-43 mRNA translation by a mechanism involving a 5′ untranslated region G-quadruplex structure, which affects GAP-43 function, as demonstrated by a GAP-43–dependent increase in neurite length and number with hnRNP-Q1 knockdown.

Posttranscriptional regulation of gene expression by mRNA-binding proteins is critical for neuronal development and function. hnRNP-Q1 is an mRNA-binding protein that regulates mRNA processing events, including translational repression. hnRNP-Q1 is highly expressed in brain tissue, suggesting a function in regulating genes critical for neuronal development. In this study, we have identified Growth-associated protein 43 (Gap-43) mRNA as a novel target of hnRNP-Q1 and have demonstrated that hnRNP-Q1 represses Gap-43 mRNA translation and consequently GAP-43 function. GAP-43 is a neuronal protein that regulates actin dynamics in growth cones and facilitates axonal growth. Previous studies have identified factors that regulate Gap-43 mRNA stability and localization, but it remains unclear whether Gap-43 mRNA translation is also regulated. Our results reveal that hnRNP-Q1 knockdown increased nascent axon length, total neurite length, and neurite number in mouse embryonic cortical neurons and enhanced Neuro2a cell process extension; these phenotypes were rescued by GAP-43 knockdown. Additionally, we have identified a G-quadruplex structure in the 5′ untranslated region of Gap-43 mRNA that directly interacts with hnRNP-Q1 as a means to inhibit Gap-43 mRNA translation. Therefore hnRNP-Q1–mediated repression of Gap-43 mRNA translation provides an additional mechanism for regulating GAP-43 expression and function and may be critical for neuronal development.

cJun promotes CNS axon growth

A number of genes regulate regeneration of peripheral axons, but their ability to drive axon growth and regeneration in the central nervous system (CNS) remains largely untested. To address this question we overexpressed eight transcription factors and one small GTPase alone and in pairwise combinations to test whether combinatorial overexpression would have a synergistic impact on CNS neuron neurite growth. The Jun oncogene/signal transducer and activator of transcription 6 (JUN/STAT6) combination increased neurite growth in dissociated cortical neurons and in injured cortical slices. In injured cortical slices, JUN overexpression increased axon growth to a similar extent as JUN and STAT6 together. Interestingly, JUN overexpression was not associated with increased growth associated protein 43 (GAP43) or integrin alpha 7 (ITGA7) expression, though these are predicted transcriptional targets. This study demonstrates that JUN overexpression in cortical neurons stimulates axon growth, but does so independently of changes in expression of genes thought to be critical for JUNs effects on axon growth. We conclude that JUN activity underlies this CNS axonal growth response, and that it is mechanistically distinct from peripheral regeneration responses, in which increases in JUN expression coincide with increases in GAP43 expression.

Transgenic inhibition of astroglial NF-kappa B leads to increased axonal sparing and sprouting following spinal cord injury

We previously showed that Nuclear Factor kappaB (NF-kappaB) inactivation in astrocytes leads to improved functional recovery following spinal cord injury (SCI). This correlated with reduced expression of pro-inflammatory mediators and chondroitin sulfate proteoglycans, and increased white matter preservation. Hence we hypothesized that inactivation of astrocytic NF-kappaB would create a more permissive environment for axonal sprouting and regeneration. We induced both contusive and complete transection SCI in GFAP-Inhibitor of kappaB-dominant negative (GFAP-IkappaBalpha-dn) and wild-type (WT) mice and performed retrograde [fluorogold (FG)] and anterograde [biotinylated dextran amine (BDA)] tracing 8 weeks after injury. Following contusive SCI, more FG-labeled cells were found in motor cortex, reticular formation, and raphe nuclei of transgenic mice. Spared and sprouting BDA-positive corticospinal axons were found caudal to the lesion in GFAP-IkappaBalpha-dn mice. Higher numbers of FG-labeled neurons were detected immediately rostral to the lesion in GFAP-IkappaBalpha-dn mice, accompanied by increased expression of synaptic and axonal growth-associated molecules. After transection, however, no FG-labeled neurons or BDA-filled axons were found rostral and caudal to the lesion, respectively, in either genotype. These data demonstrated that inhibiting astroglial NF-kappaB resulted in a growth-supporting terrain promoting sparing and sprouting, rather than regeneration, of supraspinal and propriospinal circuitries essential for locomotion, hence contributing to the improved functional recovery observed after SCI in GFAP-IkappaBalpha-dn mice.

The transcription factor ATF-3 promotes neurite outgrowth

Dorsal root ganglion (DRG) neurons regenerate after a peripheral nerve injury but not after injury to their axons in the spinal cord. A key question is which transcription factors drive the changes in gene expression that increase the intrinsic growth state of peripherally injured sensory neurons? A prime candidate is activating transcription factor-3 (ATF-3), a transcription factor that we find is induced in all DRG neurons after peripheral, but not central axonal injury. Moreover, we show in adult DRG neurons that a preconditioning peripheral, but not central axonal injury, increases their growth, correlating closely with the pattern of ATF-3 induction. Using viral vectors, we delivered ATF-3 to cultured adult DRG neurons and find that ATF-3 enhances neurite outgrowth. Furthermore, ATF-3 promotes long sparsely branched neurites. ATF-3 overexpression did not increase c-Jun expression. ATF-3 may contribute, therefore, to neurite outgrowth by orchestrating the gene expression responses in injured neurons.

CNS regeneration: clinical possibility or basic science fantasy?

Following injury to the CNS, severed axons undergo a phase of abortive sprouting in the vicinity of the wound, but do not spontaneously re-grow or regenerate. From a long history of attempts to stimulate regeneraion, a major strategy that has been developed clinically is the implantation of tissue into denervated target regions. Unfortunately trials have so far not borne out the promise that this would prove a useful therapy for disorders such as Parkinson's disease. Many strategies have also been developed to stimulate the regeneration of axons across sites of injury, particularly in the spinal cord. Animal data have demonstrated that some of these approaches hold promise and that the spinal cord has a remarkable degree of intrinsic plasticity. Attempts are now being made to utilize experimental techniques in spinal patients.

Corticospinal neurons up-regulate a range of growth-associated genes following intracortical, but not spinal, axotomy

The failure of some CNS neurons to up-regulate growth-associated genes following axotomy may contribute to their failure to regenerate axons. We have studied gene expression in rat corticospinal neurons following either proximal (intracortical) or distal (spinal) axotomy. Corticospinal neurons were retrogradely labelled with cholera toxin subunit B prior to intracortical lesions or concomitantly with spinal lesions. Alternate sections of forebrain were immunoreacted for cholera toxin subunit B or processed for mRNA in situ hybridization for ATF3, c-jun, GAP-43, CAP-23, SCG10, L1, CHL1 or krox-24, each of which has been associated with axotomy or axon regeneration in other neurons. Seven days after intracortical axotomy, ATF3, c-jun, GAP-43, SCG10, L1 and CHL1, but not CAP-23 or krox-24, were up-regulated by layer V pyramidal neurons, including identified corticospinal neurons. The maximum distance between the lesion and the neuronal cell bodies that up-regulated genes varied between 300 and 500 microm. However, distal axotomy failed to elicit changes in gene expression in corticospinal neurons. No change in expression of any molecule was seen in the neocortex 1 or 7 days after corticospinal axotomy in the cervical spinal cord. The expression of GAP-43, CAP-23, L1, CHL1 and SCG10 was confirmed to be unaltered after this type of injury in identified retrogradely labelled corticospinal neurons. Thus, while corticospinal neuronal cell bodies fail to respond to spinal axotomy, these cells behave like regeneration-competent neurons, up-regulating a wide range of growth-associated molecules if axotomized within the cerebral cortex.

A new millenium for spinal cord regeneration: growth-associated genes

INTRODUCTION

Neurons surviving spinal cord injury undergo extensive reorganization that may result in the formation of functional synaptic contacts. Many neurons, however, fail to activate the necessary mechanisms for successful regeneration. In this review, we discuss the implications of growth cone genes that we have correlated with successful spinal cord axonal regeneration.

METHOD

Factors that inhibit regeneration, and activation of genes that promote it are discussed.

RESULTS/DISCUSSION

The early progress n understanding mechanisms that seem to promote or inhibit regeneration in the central nervous system may have significant clinical utility in the future.

Retrograde repression of growth-associated protein-43 mRNA expression in rat cortical neurons

Corticospinal neurons support rapid growth of axons toward spinal cord targets in the perinatal period. Initial axon growth is accompanied by elevated expression of growth-associated protein-43 (GAP-43), which then declines in postnatal development. To investigate whether expression of GAP-43 mRNA is regulated by retrograde signals, we injected colchicine into the corticospinal tract to block retrograde axonal transport during a time when GAP-43 is normally declining in corticospinal neurons. Colchicine caused a prolongation of high GAP-43 mRNA expression in neurons located in layer V (but not other layers) of sensorimotor cortex. We next used osmotic minipumps to infuse soluble adult spinal cord extract into the sensorimotor cortex. This resulted in a premature downregulation of GAP-43 mRNA in identified corticospinal neurons. GAP-43 repressive activity was found in extracts of the spinal cord tissue as young as postnatal day 8. The effect of spinal cord extract in vivo was not mimicked by adult cerebellar or muscle extracts. Cultures of postnatal cortical neurons also underwent downregulation of GAP-43 mRNA when treated with spinal cord extract. Activation of cAMP signaling also repressed GAP-43 mRNA in cortical cultures, and the repressive effect of spinal cord extract was diminished by an adenyl cyclase inhibitor. Thus, GAP-43 mRNA may be downregulated late in development by a target-derived retrograde repressive factor, and this effect may be mediated by cAMP second messenger signaling.

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

In contrast to peripheral nerves, damaged axons in the mammalian brain and spinal cord rarely regenerate. Peripheral nerve injury stimulates neuronal expression of many genes that are not generally induced by CNS lesions, but it is not known which of these genes are required for regeneration. Here we show that co-expressing two major growth cone proteins, GAP-43 and CAP-23, can elicit long axon extension by adult dorsal root ganglion (DRG) neurons in vitro. Moreover, this expression triggers a 60-fold increase in regeneration of DRG axons in adult mice after spinal cord injury in vivo. Replacing key growth cone components, therefore, could be an effective way to stimulate regeneration of CNS axons.

Up-regulation of growth-associated protein 43 mRNA in rat medial septum neurons axotomized by fimbria-fornix transection

Transection of septohippocampal fibres is widely used to study the response of CNS neurons to axotomy. Septohippocampal projection neurons survive axotomy and selectively up-regulate the transcription factor c-Jun. In the present study we investigated whether these cells concomitantly up-regulate the growth-associated protein-43 (GAP-43), a potential target gene of c-Jun implicated in axonal growth and regeneration. Using in situ hybridization histochemistry (ISHH) it was demonstrated that postlesional c-jun mRNA expression is accompanied by an increased expression of GAP-43 mRNA in the medial septum 3 days following fimbria-fornix transection (FFT). The increase reached a maximum at 7 days and gradually declined thereafter (17 days, 3 weeks). Retrograde prelabeling with Fluoro-Gold followed by axotomy and ISHH revealed that GAP-43 mRNA was up-regulated in septohippocampal projection neurons. Colocalization of GAP-43 mRNA and choline acetyltransferase protein showed that GAP-43 mRNA was expressed by cholinergic medial septal neurons after axotomy. Selective immunolesioning of the cholinergic component of the septohippocampal projection with 192 IgG-saporin followed by FFT demonstrated that GAP-43 mRNA was also synthesized by axotomized GABAergic neurons. These results demonstrate an up-regulation of GAP-43 mRNA in axotomized septohippocampal projection neurons independent of their transmitter phenotype which is closely correlated with c-Jun expression. Because the GAP-43 gene contains an AP-1 site, we hypothesize a c-Jun-driven up-regulation of GAP-43 in lesioned medial septal neurons that may contribute to their survival and regenerative potential following axotomy.

The cell recognition molecule CHL1 is strongly upregulated by injured and regenerating thalamic neurons

Close homologue of L1 (CHL1) is a cell recognition molecule known to promote axonal growth in vitro. We have investigated the expression of CHL1 mRNA by regenerating central nervous system (CNS) neurons, by using in situ hybridisation 3 days to 10 weeks following the implantation of living and freeze-killed peripheral nerve autografts into the thalamus of adult rats. At all survival times after implantation of living grafts, neurons of the thalamic reticular nucleus (TRN), close to the graft tip and up to 1 mm away from it, displayed strong signal for CHL1 mRNA, even though TRN neurons show very low levels of CHL1 mRNA expression in unoperated animals. When the cell bodies of regenerating neurons were identified by retrograde labelling from the distal portion of the grafts, 4-6 weeks after operation, most of the labelled cells were found in the TRN and could be shown to haveupregulated CHL1 mRNA. In addition, some neurons in dorsal thalamic nuclei near the graft tip transiently upregulated CHL1 mRNA during the first 3 weeks after graft implantation, and glial cells showing CHL1 mRNA expression were present at the brain/graft interface 3 days to 2 weeks after operation. Freeze-killed grafts, into which axons do not regenerate, caused a transient upregulation of CHL1 in very few TRN neurons near the graft tip and in glial cells at the brain/graft interface but did not produce prolonged CHL1 mRNA expression. CHL1 can therefore be added to the list of molecules (including GAP-43, L1, and c-jun) strongly expressed by CNS neurons that regenerate their axons into nerve grafts, but not by those neurons that fail to regenerate their axons.

Overexpression of GAP-43 in thalamic projection neurons of transgenic mice does not enable them to regenerate axons through peripheral nerve grafts

It is well established that some populations of neurons of the adult rat central nervous system (CNS) will regenerate axons into a peripheral nerve implant, but others, including most thalamocortical projection neurons, will not. The ability to regenerate axons may depend on whether neurons can express growth-related genes such as GAP-43, whose expression correlates with axon growth during development and with competence to regenerate. Thalamic projection neurons which fail to regenerate into a graft also fail to upregulate GAP-43. We have tested the hypothesis that the absence of strong GAP-43 expression by the thalamic projection neurons prevents them from regenerating their axons, using transgenic mice which overexpress GAP-43. Transgene expression was mapped by in situ hybridization with a digoxigenin-labeled RNA probe and by immunohistochemistry with a monoclonal antibody against the GAP-43 protein produced by the transgene. Many CNS neurons were found to express the mRNA and protein, including neurons of the mediodorsal and ventromedial thalamic nuclei, which rarely regenerate axons into peripheral nerve grafts. Grafts were implanted into the region of these nuclei in the brains of transgenic animals. Although these neurons strongly expressed the transgene mRNA and protein and transported the protein to their axon terminals, they did not regenerate axons into the graft, suggesting that lack of GAP-43 expression is not the only factor preventing thalamocortical neurons regenerating their axons.

Intercostal nerve implants transduced with an adenoviral vector encoding neurotrophin-3 promote regrowth of injured rat corticospinal tract fibers and improve hindlimb function

Following injury to central nervous tissues, damaged neurons are unable to regenerate their axons spontaneously. Implantation of peripheral nerves into the CNS, however, does result in axonal regeneration into these transplants and is one of the most powerful strategies to promote CNS regeneration. In the present study implantation of peripheral nerve bridges following dorsal hemisection is combined with ex vivo gene transfer with adenoviral vectors encoding neurotrophin-3 (Ad-NT-3) to examine whether this would stimulate regeneration of one of the long descending tracts of the spinal cord, the corticospinal tract (CST), into and beyond the peripheral nerve implant. We chose to use an adenoviral vector encoding NT-3 because CST axons are sensitive to this neurotrophin and Schwann cells in peripheral nerve implants do not express this neurotrophin. At 16 weeks postimplantation of Ad-NT-3-transduced intercostal nerves, approximately three- to fourfold more of the anterogradely traced corticospinal tract fibers had regrown their axons through gray matter below the lesion site when compared to control animals. Regrowth of CST fibers occurred over more than 8 mm distal to the lesion site. No regenerating CST fibers were, however, observed into the transduced peripheral implant. Animals with a peripheral nerve transduced with Ad-NT-3 also exhibited improved function of the hindlimbs when compared to control animals treated with an adenoviral vector encoding LacZ. Thus, transient overexpression of NT-3 in peripheral nerve tissue bridges is apparently sufficient to stimulate regrowth of CST fibers and to promote recovery of hindlimb function, but does not result in regeneration of CST fibers into such transplants. Taken together, combining an established neurotransplantation approach with viral vector-gene transfer promotes the regrowth of injured CST fibers through gray matter and improves the recovery of hindlimb function.

Expression of CHL1 and L1 by neurons and glia following sciatic nerve and dorsal root injury

Cell adhesion molecules (CAMs), particularly L1, are important for axonal growth on Schwann cells in vitro. We have used in situ hybridization to study the expression of mRNAs for L1 and its close homologue CHL1, by neurons regenerating their axons in vivo, and have compared CAM expression with that of GAP-43. Adult rat sciatic nerves were crushed (allowing functional regeneration), or cut and ligated to maintain axonal sprouting but prevent reconnection with targets. In other animals lumbar dorsal roots were transected to produce slow regeneration of the central axons of sensory neurons. In unoperated animals L1 and CHL1 mRNAs were expressed at moderate levels by small- to medium-sized sensory neurons and L1 mRNA was expressed at moderate levels by motor neurons. Many large sensory neurons expressed neither L1 nor CHL1 mRNAs and motor neurons expressed little or no CHL1 mRNA. Neither motor nor sensory neurons showed any obvious upregulation of L1 mRNA after axotomy. Increased CHL1 mRNA was found in motor neurons and small- to medium-sized sensory neurons 3 days to 2 weeks following sciatic nerve crush, declining toward control levels by 5 weeks when regeneration was complete. Cut and ligation injuries caused a prolonged upregulation of CHL1 mRNA (and GAP-43 mRNA), indicating that reconnection with target tissues may be required to signal the return to control levels. Large sensory neurons did not upregulate CHL1 mRNA after axotomy and thus regenerated within the sciatic nerve without producing CHL1 or L1. Dorsal root injuries caused a modest, slow upregulation of CHL1 mRNA by some sensory neurons. CHL1 mRNA was also upregulated by many presumptive Schwann cells in injured nerves and by some satellite cells around large sensory neurons after sciatic nerve injuries and was transiently upregulated by some astrocytes in the degenerating dorsal columns after dorsal rhizotomy.

Influence of the axotomy to cell body distance in rat rubrospinal and spinal motoneurons: differential regulation of GAP-43, tubulins, and neurofilament-M

Axotomized motoneurons regenerate their axons regardless of whether axotomy occurs proximally or distally from their cell bodies. In contrast, regeneration of rubrospinal axons into peripheral nerve grafts has been detected after cervical but not after thoracic injury of the rubrospinal tract. By using in situ hybridization (ISH) combined with reliable retrograde tracing methods, we compared regeneration-associated gene expression after proximal and distal axotomy in spinal motoneurons versus rubrospinal neurons. Regardless of whether they were axotomized at the iliac crest (proximal) or popliteal fossa (distal), sciatic motoneurons underwent highly pronounced changes in ISH signals for Growth Associated Protein 43 (GAP-43) (10-20x increase) and neurofilament M (60-85% decrease). In contrast, tubulin ISH signals substantially increased only after proximal axotomy (3-5x increase). To compare these changes in gene expression with those of axotomized rubrospinal neurons, the rubrospinal tract was transected at the cervical (proximal) or thoracic (distal) levels of the spinal cord. Cervically axotomized rubrospinal neurons showed three- to fivefold increases in ISH signals for GAP-43 and tubulins (only transient) and a 75% decrease for neurofilament-M. In sharp contrast, thoracic axotomy had only marginal effects. After implantation of peripheral nerve transplants into the spinal cord injury sites, retrograde labeling with the sensitive retrograde tracer Fluoro-Gold identified regenerating rubrospinal neurons only after cervical axotomy. Furthermore, rubrospinal neurons specifically regenerating into the transplants were hypertrophied and expressed high levels of GAP-43 and tubulins. Taken together, these data support the concept that, even if central nervous system (CNS) axons are presented with a permissive/supportive environment, appropriate cell body responses to injury are a prerequisite for CNS axonal regeneration.

Cellular and molecular correlates of the regeneration of adult mammalian CNS axons into peripheral nerve grafts

Studies of the regeneration of CNS axons into peripheral nerve grafts have provided information crucial to our understanding of the regenerative potential of CNS neurons. Injured axons in the thalamus and corpus striatum produce regenerative sprouts within a few days of graft implantation, apparently in response to living cells in the grafts. The regenerating axons often grow directly towards the grafts, and enter Schwann cell columns where they elongate surrounded by Schwann cell processes. The regenerating CNS axons, and the Schwann cell processes along which they grow, initially express the cell adhesion molecules NCAM, and L1. The axons also express polysialic acid and, unlike regenerating peripheral axons, bind tenascin-C derived from Schwann cells. Wherever peripheral nerve grafts are implanted into the CNS they appear to promote the differential regeneration of CNS axons. Most of the axons which grow into grafts in the thalamus are derived from the thalamic reticular nucleus (TRN), whereas grafts in the striatum promote regeneration of axons from the substantia nigra pars compacta (SNpc) and grafts in the cerebellum promote regeneration from deep cerebellar nuclei (DCN) and brainstem precerebellar neurons. In contrast most thalamocortical projection neurons, striatal projection neurons and Purkinje cells in the cerebellar cortex are poor at regenerating. There are patterns to the expression of regeneration-related molecules by axons injured by nerve grafts in the CNS. Most neurons which regenerate well (e.g. TRN and DCN neurons) upregulate GAP-43, L1 and the transcription factor c-jun in response to a graft, whereas those neurons which do not regenerate well (e.g. Purkinje cells, thalamocortical and striatal projection neurons) do not upregulate these molecules. These observations suggest that some classes of CNS neurons may be intrinsically unable to regenerate axons and the repair of injuries in the brain and spinal cord may consequently require some form of gene therapy for axotomised neurons.

Growth-associated protein-43 is increased in cerebrum of immature rats following induction of hydrocephalus

Hydrocephalus is associated with gradual progressive impairment and destruction of cerebral axons and neurons. Growth associated protein-43 appears to be permissive for neuro-axonal regeneration and synaptic remodeling. Hydrocephalus was induced in three-week-old rats by injection of kaolin into the cisterna magna. Compared to controls, cerebral growth-associated protein-43 messenger RNA was significantly up-regulated one week after kaolin injection and the overall cerebral growth-associated protein-43 protein level was significantly higher at four weeks when the ventricles were severely enlarged. One and three weeks after kaolin injection, growth-associated protein-43-like immunoreactivity was increased in periventricular axons, and also in the cerebral cortex at three weeks. Hydrocephalic rats that had been treated by shunting after one week, exhibited growth-associated protein-43 messenger RNA and protein levels intermediate between hydrocephalic rats and control rats. The increase in periventricular axon growth-associated protein-43, early in the course of experimental hydrocephalus, suggests that through early intervention there may be a chance for preventing or reversing the axonal injury. Cortical expression of growth associated protein-43 suggests that an alteration in synaptogenesis may also occur.

The angiotensin II type 2 (AT2) receptor promotes axonal regeneration in the optic nerve of adult rats

The renin-angiotensin system (RAS) has been traditionally linked to blood pressure and volume regulation mediated through the angiotensin II (ANG II) type 1 (AT1) receptor. Here we report that ANG II via its ANG II type 2 (AT2) receptor promotes the axonal elongation of postnatal rat retinal explants (postnatal day 11) and dorsal root ganglia neurons in vitro, and, moreover, axonal regeneration of retinal ganglion cells after optic nerve crush in vivo. In retinal explants, ANG II (10(-7)-10(-5) M) induced neurite elongation via its AT2 receptor, since the effects were mimicked by the AT2 receptor agonist CGP 42112 (10(-5) M) and were entirely abolished by costimulation with the AT2 receptor antagonist PD 123177 (10(-5) M), but not by the AT1 receptor antagonist losartan (10(-5) M). To investigate whether ANG II is able to promote axonal regeneration in vivo, we performed optic nerve crush experiments in the adult rats. After ANG II treatment (0.6 nmol), an increased number of growth-associated protein (GAP)-43-positive fibers was detected and the regenerating fibers regularly crossed the lesion site (1.6 mm). Cotreatment with the AT2 receptor antagonist PD 123177 (6 nmol), but not with the AT1 receptor antagonist losartan (6 nmol), completely abolished the ANG II-induced axonal regeneration, providing for the first time direct evidence for receptor-specific neurotrophic action of ANG II in the central nervous system of adult mammals and revealing a hitherto unknown function of the RAS.

Axonal injury and peripheral nerve grafting in the thalamus and cerebellum of the adult rat: upregulation of c-jun and correlation with regenerative potential

The protooncogene c-jun is highly expressed for long periods in axotomized PNS neurons. This may be related to their growth and regeneration. In contrast, axotomized CNS neurons show only a small and transient upregulation of c-jun. It has been suggested that there may be a correlation between this failure to maintain high levels of c-jun expression after axotomy and abortive CNS axonal regeneration. We have studied, by in situ hybridization and immunohistochemistry, the c-jun response after stab wound lesion, and after peripheral nerve grafting in the thalamus and cerebellum of the adult rat. A lesion elicits upregulation of c-jun in thalamic neurons ipsilateral to the lesion. This is most evident and prolonged in neurons such as those of the thalamic reticular nucleus, which have an established propensity to regenerate. After peripheral nerve grafting, the c-jun response in thalamic neurons is enhanced, mostly in neurons which have axons regenerating along the grafts. These neurons also upregulate growth-associated protein 43 (GAP-43). By comparison, injured Purkinje cells of the cerebellum which do not regenerate their axons along a graft, do not upregulate either c-jun or GAP-43, although they increase their expression of p75. Thus CNS neurons able to regenerate their axons along a peripheral nerve graft are those in which c-jun is induced after injury, and c-jun may play a critical role in the control of gene programs for axonal regeneration. Moreover, the observed differences in the ability of CNS neurons to regenerate their axons may relate to a difference in their intrinsic molecular response to axotomy.

Peripheral nerve grafts exert trophic and tropic effects on anterior thalamic neurons

Peripheral nerve grafting into the central nervous system (CNS) has been used to study the regenerative capabilities of central neurons given access to a peripheral nervous system (PNS) environment. It is well documented that many CNS neurons regenerate axons along peripheral nerve grafts placed in close proximity to their cell bodies and that these grafts can ameliorate axotomy-induced retrograde degeneration. In the present study, we placed peripheral nerve grafts in proximity to axotomized neurons of the anterior thalamus. Standard histological and retrograde tracing techniques were used to examine these preparations 2 months after grafting. Three effects of these grafts were observed: amelioration of retrograde degeneration of axotomized anterior thalamic neurons, hypertrophy of many thalamic neurons in the local environment of the graft, and ingrowth of axons of axotomized anterior thalamic neurons as well as nonaxotomized neurons from surrounding nuclei. We conclude from these studies that peripheral nerve grafts not only provide a matrix for axonal outgrowth but also exert marked trophic and tropic effects on axotomized anterior thalamic neurons.

Targeted overexpression of the neurite growth-associated protein B-50/GAP-43 in cerebellar Purkinje cells induces sprouting after axotomy but not axon regeneration into growth-permissive transplants

B-50/GAP-43 is a nervous tissue-specific protein, the expression of which is associated with axon growth and regeneration. Its overexpression in transgenic mice produces spontaneous axonal sprouting and enhances induced remodeling in several neuron populations (; ). We examined the capacity of this protein to increase the regenerative potential of injured adult central axons, by inducing targeted B-50/GAP-43 overexpression in Purkinje cells, which normally show poor regenerative capabilities. Thus, transgenic mice were produced in which B-50/GAP-43 overexpression was driven by the Purkinje cell-specific L7 promoter. Uninjured transgenic Purkinje cells displayed normal morphology, indicating that transgene expression does not modify the normal phenotype of these neurons. By contrast, after axotomy numerous transgenic Purkinje cells exhibited profuse sprouting along the axon and at its severed end. Nevertheless, despite these growth phenomena, which never occurred in wild-type mice, the severed transgenic axons were not able to regenerate, either spontaneously or into embryonic neural or Schwann cell grafts placed into the lesion site. Finally, although only a moderate Purkinje cell loss occurred in wild-type cerebella after axotomy, a considerable number of injured transgenic neurons degenerated, but they could be partially rescued by the different transplants placed into the lesion site. Thus, B-50/GAP-43 overexpression substantially modifies Purkinje cell response to axotomy, by inducing growth processes and decreasing their resistance to injury. However, the presence of this protein is not sufficient to enable these neurons to accomplish a full program of axon regeneration.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

A transcription-dependent switch controls competence of adult neurons for distinct modes of axon growth

Although maturing neurons undergo a precipitous decline in the expression of genes associated with developmental axon growth, structural changes in axon arbors occur in the adult nervous system under both normal and pathological conditions. Furthermore, some neurons support extensive regrowth of long axons after nerve injury. Analysis of adult dorsal root ganglion (DRG) neurons in culture now shows that competence for distinct types of axon growth depends on different patterns of gene expression. In the absence of ongoing transcription, newly isolated neurons can extend compact, highly branched arbors during the first day in culture. Neurons subjected to peripheral axon injury 2-7 d before plating support a distinct mode of growth characterized by rapid extension of long, sparsely branched axons. A transition from "arborizing" to "elongating" growth occurs in naive adult neurons after approximately 24 hr in culture but requires a discrete period of new transcription after removal of the ganglia from the intact animal. Thus, peripheral axotomy-by nerve crush or during removal of DRGs--induces a transcription-dependent change that alters the type of axon growth that can be executed by these adult neurons. This transition appears to be triggered, in large part, by interruption of retrogradely transported signals, because blocking axonal transport in vivo can elicit competence for elongating growth in many DRG neurons. In contrast to peripheral axotomy, interruption of the centrally projecting axons of DRG neurons in vivo leads to subsequent growth in vitro that is intermediate between "arborizing" and "elongating" growth. This suggests that the transition between these two modes of growth is a multistep process and that individual steps may be regulated separately. These observations together suggest that structural remodeling in the adult nervous system need not involve the same molecular apparatus as long axon growth during development and regeneration.

GAP-43: an intrinsic determinant of neuronal development and plasticity

Several lines of investigation have helped clarify the role of GAP-43 (FI, B-50 or neuromodulin) in regulating the growth state of axon terminals. In transgenic mice, overexpression of GAP-43 leads to the spontaneous formation of new synapses and enhanced sprouting after injury. Null mutation of the GAP-43 gene disrupts axonal pathfinding and is generally lethal shortly after birth. Manipulations of GAP-43 expression likewise have profound effects on neurite outgrowth for cells in culture. GAP-43 appears to be involved in transducing intra- and extracellular signals to regulate cytoskeletal organization in the nerve ending. Phosphorylation by protein kinase C is particularly significant in this regard, and is linked with both nerve-terminal sprouting and long-term potentiation. In the brains of humans and other primates, high levels of GAP-43 persist in neocortical association areas and in the limbic system throughout life, where the protein might play an important role in mediating experience-dependent plasticity.

Intrinsic versus extrinsic factors in determining the regeneration of the central processes of rat dorsal root ganglion neurons: the influence of a peripheral nerve graft

The relative contribution of intrinsic growth capacity versus extrinsic growth-promoting factors in determining the capacity of transected dorsal root axons to regenerate long distances was studied. L4 dorsal root axons regenerating into 4-cm peripheral nerve grafts on transected dorsal roots were counted. Few dorsal root myelinated axons regenerated to the distal end of the grafts by 10 weeks unless the sciatic nerve was also crushed. Regeneration of unmyelinated axons was also increased by peripheral lesions. Crush or transection of the dorsal roots without grafting did not alter GAP-43 mRNA expression in L4 dorsal root ganglion (DRG) cells. Grafting a peripheral nerve onto the cut end of an L4 dorsal root doubled the number of DRG cells expressing high levels of GAP-43 mRNA after a delay of several weeks. Peripheral nerve crush at the time of nerve grafting resulted in a very rapid rise in GAP-43 mRNA expression, which then declined to a steady level, twice that of controls, by 7 weeks. Thus, the rapid increase in the number of DRG neurons expressing high levels of GAP-43 mRNA after peripheral but not central axotomy correlates with the regeneration of central axons through nerve grafts. Because GAP-43 mRNA is slowly upregulated in a subpopulation of sensory neurons in response to exposure of their central axons to a peripheral nerve environment, environments favourable for axonal growth may act by increasing the intrinsic growth response of neurons. Lack of intrinsic growth capacity may contribute to the failure of dorsal root axons to regenerate into the spinal cord.

Axonal regeneration

Axons damaged in a peripheral nerve are often able to regenerate from the site of injury along the degenerate distal segment of the nerve to reform functional synapses. Schwann cells play a central role in this process. However, in the adult mammalian central nervous system, from which Schwann cells are absent, axonal regeneration does not progress to allow functional recovery. This is due to inhibitors of axonal growth produced by both oligodendrocytes and astrocytes and also to the decreased ability of adult neurons to extend axons during regeneration compared to embryonic neurons during development. However once provided with a substrate conducive to axonal growth, such as a peripheral nerve graft, many central neurons are able to regenerate axons over long distances. Over the past year this response has been utilised in experimental models to produce a degree of behavioural recovery.

Rapid induction by kainic acid of both axonal growth and F1/GAP-43 protein in the adult rat hippocampal granule cells

Hippocampal granule cells do not normally express the axonal growth- and plasticity-associated protein F1/GAP-43 in the adult rat. Using three different methods that lead to hypersynchronous activity in limbic circuits, expression of F1/GAP-43 mRNA can be induced in granule cells which is followed by sprouting in mossy fibers, the axons of granule cells. F1/GAP-43 mRNA expression in granule cells was induced in the temporal, but not septal, hippocampus beginning at 12 hours after kainic acid (KA) subcutaneous injection (10 mg/kg). Beginning 2 days after KA treatment, mossy fiber sprouts restricted to the temporal hippocampus were observed in the supragranular layer. In the same animal we also observed that levels of protein F1/GAP-43 immunoreactivity in this layer apparently increased at this same 2 day time point and same ventral hippocampal location. F1/GAP-43 protein levels and mossy fiber sprouting showed an increase up to 10 days after KA treatment. Sprouting was at a maximum at 40 days, the longest time point studied. These events parallel axonal regeneration with one critical difference: granule cell axons are not damaged by kainate. The rapid onset of axonal growth in the adult is striking and occurs earlier than reported previously (2 days vs. 12 days). Such growth closely associated with elevated levels of protein F1/GAP-43 may occur as a result of a) reactive synaptogenesis caused by the availability of post-synaptic surface on granule cell dendrites at the supragranular layer, b) Hebbian co-activation of the post-synaptic granule cells and their presynaptic afferents, and c) loss of target-derived inhibitory growth factor.

Influence of peripheral nerve grafts on the expression of GAP-43 in regenerating retinal ganglion cells in adult hamsters

We have examined the ability of axotomized retinal ganglion cells in adult hamsters, to regenerate axons into a peripheral nerve graft attached to the optic nerve and the expression of GAP-43 by these neurons. We also examined the effect on these events of transplanting a segment of peripheral nerve to the vitreous body. The left optic nerves in three groups of hamsters were replaced with a long segment of peripheral nerve attached to the proximal stump of the optic nerve approximately 2 mm from the optic disc to induce regeneration of retinal ganglion cells into the peripheral nerve. An additional segment of peripheral nerve was transplanted into the vitreous of the left eye in the second group. The animals from the first and second groups were allowed to survive for 1-8 weeks and the number of regenerating retinal ganglion cells was determined by applying the retrograde tracer, Fluoro-Gold to the peripheral nerve graft and the expression of GAP-43 was studied by immunocytochemistry in the same retinas. As a control, a segment of optic nerve was transplanted into the vitreous body of the left eye in the third group of hamsters. These animals were allowed to survive for 4 weeks and the number of regenerating retinal ganglion cells was counted as in Groups 1 and 2. The percentages of the regenerating retinal ganglion cells which also expressed GAP-43 were very high at all time points in Group 1 (with no intravitreal peripheral nerve) and Group 2 (with intravitreal peripheral nerve) and at 4 weeks for the Group 3 (with intravitreal optic nerve) animals. In addition, the number of regenerating retinal ganglion cells, the number of retinal ganglion cells expressing GAP-43 and the number of regenerating retinal ganglion cells which also expressed GAP-43 were much higher in Group 2 than in Group 1 at all the time points and it was also much higher in Group 2 than in Group 3 at 4 weeks whereas there was no significant difference between the results from Groups 1 and 3 at 4 weeks. These data suggested that there was a close correlation between the number of the axotomized retinal ganglion cells regenerating axons into the peripheral nerve graft attached to the optic nerve and the expression of GAP-43.(ABSTRACT TRUNCATED AT 400 WORDS)

Identified cholinergic neurones in the adult rat brain are enriched in GAP-43 mRNA: a double in situ hybridisation study

The cellular expression of growth associated protein-43 mRNA by identified choline acetyl transferase mRNA positive cells was investigated in the mature rat brain using a combined radioactive and non-radioactive in situ hybridisation technique. Cellular sites of growth associated protein-43 mRNA were detected using a 35S-oligonucleotide while choline acetyl transferase mRNA positive neurones were identified using two alkaline phosphatase-labelled probes. In the cholinergic cells of the corpus striatum, basal forebrain and laterodorsal tegmental nucleus a specific growth associated protein-43 hybridisation signal (silver grains) was detected, demonstrating that these choline acetyl transferase mRNA positive cells are enriched in growth associated protein-43 gene transcripts. By contrast, the large cholinergic cells of the motor nucleus of the trigeminal nerve did not express growth associated protein-43 mRNA. Quantification of the growth associated protein-43 hybridisation signal expressed by identified choline acetyl transferase mRNA positive cells showed regional variations in the relative cellular abundance of this transcript; cholinergic cells in the laterodorsal tegmental nucleus and corpus striatum expressed the strongest cellular hybridisation signal. Mean cross-sectional somatic area measurements of these growth associated protein-43/cholinergic positive cells confirmed the identity of these neurones as belonging to the cholinergic phenotype. A strong 35S-growth associated protein-43 hybridisation signal was detected also in numerous other non-choline acetyl transferase mRNA positive nerve cells in other regions of the brain, although the chemical phenotypes of these neurones were not determined. Our data reveal that expression of the growth-associated protein GAP-43 is maintained in identified cholinergic neurones in the postnatal rat brain, suggesting that this protein may subserve important functions in cholinergic and other neurones of the adult mammalian brain.

Lesion-specific pattern of immunocytochemical distribution of growth-associated protein B-50 (GAP-43) in the cerebellum of Weaver and PCD-mutant mice: lack of B-50 involvement in neuroplasticity of Purkinje cell terminals?

The growth-associated protein B-50 (GAP-43) is thought to play a major role in the development and regeneration of neurons. The participation of B-50 in neuronal plasticity is well documented, especially for monoaminergic systems. However, such an important role for B-50 in GABAergic systems has not been substantiated to date. This study was performed to obtain detailed information about the identity of B-50 immunopositive axons and terminals in the cerebellum and to test the involvement of this protein during plastic changes as observed in the projections of GABAergic Purkinje cells to the lateral vestibular nucleus (LVN). For this purpose mutant mice with specific cerebellar cell loss were used. Weaver mutants (B6CBA wv/wv), PCD-mutants (B6C3Fe pcd/pcd), and their corresponding wild-type mice were investigated with immunocytochemical and immunoblot procedures at the age of 8-23 days and 5-6 months using polyclonal and monoclonal antibodies to B-50. Substantial differences in B-50 distribution were detected between normals and mutants and between young and adult animals. These results demonstrate that the labeling of B-50 is mainly related to the out-growth of parallel fibers and to a minor degree on the ingrowth of non-GABAergic cerebellar afferents. There was no immunocytochemical indication that B-50 is related to Purkinje cells or accompanies the plasticity of the GABAergic innervation of the LVN.

Regional distribution of amyloid beta-protein precursor, growth-associated phosphoprotein-43 and microtubule-associated protein 2 messenger RNAs in the nigrostriatal system of normal and Weaver mutant mice and effects of ventral mesencephalic grafts

Using in situ hybridization histochemistry with [32P]oligonucleotide probes, we studied the cellular localization of RNA transcripts for amyloid beta-protein precursor (beta APP), growth-associated phosphoprotein-43 (GAP-43) and microtubule-associated protein 2 (MAP2) in the mesostriatal system of normal (+/+) and weaver (wv/wv) mutant mice, which lose mesencephalic dopamine neurons. In addition, expression of the same messages was studied in ventral mesencephalic cell suspensions transplanted to the weaver striatum. Transcripts encoding GAP-43, MAP2 and isoforms beta APP695, beta APP714 and beta APP751 were present in normal substantia nigra and progressively reduced in weaver substantia nigra; such a reduction was correlated with dopamine neuron loss. The survival of dopamine neurons in unilateral intrastriatal grafts was documented by methamphetamine-induced rotational asymmetry tests and by tyrosine hydroxylase immunocytochemistry. High hybridization signals were obtained for GAP-43, MAP2, beta APP695, beta APP714 and beta APP751 RNA transcripts in the grafted tissue; the beta APP770 species--normally seen in striatum and not substantia nigra--was not expressed in the grafts, but it was present in the recipient striatum. Following immunocytochemical labelling with antibodies, GAP-43 and MAP2 immunoreactivities were seen in cell processes in the grafts and surrounding tissue, whereas beta APP immunoreactivity was mainly found in grafted cell bodies. These results suggest that the transplanted mesencephalic cells mature very similarly to those in the normal substantia nigra, expressing different mRNAs that are normally present in the ventral midbrain and which are reduced in the weaver mutant as a consequence of dopamine neuron loss.

Distribution of GAP-43 in relation to CGRP and synaptic vesicle markers in rat skeletal muscles during development

GAP 43 in nerve terminal structures of rat skeletal muscles, was investigated during postnatal development using immunofluorescence and confocal laser scanning microscopy. Comparison with synaptophysin, synapsin, SV2, CGRP, SP and NF was done in double immunoincubation studies. GAP 43-like immunoreactivity (LI) was demonstrated in preterminal axons and motor endplates in all age groups (from E18 to adult), although the intensity of immunofluorescence was considerably higher in the younger rats. The outgrowing nerve sprouts in E18 muscles were strongly GAP 43-positive. The intensity decreased with increasing age, but even in adult animals GAP 43-LI was present in some p38- or SV2-positive endplates. GAP 43-LI was also present in muscle spindles and preterminal nerve branches, and likewise decreased with age. Perivascular nerve terminals (around arteries mainly) were, however, strong in GAP 43-LI during both development and adulthood. GAP 43-LI was strong, and present in both small and large granules. SP-LI was observed in a few thin, presumably sensory, axons around vessels, which also contained a few GAP 43-positive large granules. Most of the strongly GAP 43-positive terminals around vessels were probably autonomic postganglionic terminals. The results suggest that GAP 43, in addition to development and regeneration, may play a significant role also in normal adult rats, especially in perivascular nerve terminals, possibly connected with a high potential for plasticity in this kind of nerve terminals.

Regulation of growth-associated protein 43 (GAP-43) messenger RNA associated with plastic change in the adult rat barrel receptor complex

Plastic change occurs in the adult rat barrel receptor complex following peripheral deafferentation by removal of facial vibrissae (vibrissectomy) and can be prevented by prior depletion of brain norepinephrine. Growth-associated protein (GAP-43, B50, F1, pp46), a marker for synaptic reorganization, increases in the barrel cortex of adult rats following both peripheral and central deafferentation. Here we followed changes in GAP-43 mRNA expression in the barrel receptor system following vibrissectomy. Adult rats had unilateral total vibrissectomy with sparing of the central (C3) vibrissa. By in situ hybridization, GAP-43 mRNA first increased at 24h (9%, P < 0.05) in the ipsilateral trigeminal complex. Levels remained elevated (up to 25% of the unlesioned side) over the next 6 days, decreased to 88% at 7 days and returned to control levels at 14 days. Contralateral barrel cortex levels of GAP-43 mRNA increased by 14% at 4-5 days remained elevated through 7 days and returned to control levels by 14 days. Increased GAP-43 mRNA levels 6 days after vibrissectomy were reproduced by complete transection of the infraorbital nerve and were blocked by depletion of brain norepinephrine. No change occurred in ventrobasal thalamus GAP-43 mRNA at any time. Dot blot and Northern blot hybridizations of GAP-43 mRNA after vibrissectomy showed a 43% increase in the ipsilateral trigeminal complex and a 16% increase in the contralateral barrel cortex at 3 days and an 84% increase in ipsilateral trigeminal and 50% increase in contralateral barrel cortex GAP-43 mRNA at 6 days, respectively. Thus, deafferentation-induced plasticity in the barrel pathway depends upon norepinephrine and is associated with increase in both GAP-43 mRNA and protein suggesting that this may involve a structural change.

Up-regulation of GAP-43 and growth of axons in rat spinal cord after compression injury

The growth-associated protein-43 (GAP-43) is an axonal phosphoprotein which is expressed at high levels during development and is reinduced by regeneration in the PNS. Consequently it is believed to be a key molecule in the regulation of axonal growth. However, injury to the CNS does not result in significant regeneration and this has been suggested to correlate with a failure of central neurons to up-regulate GAP-43 after axotomy. We have examined a model of spinal cord injury which is unique in two respects; first dural integrity is maintained by compression of the cord with smooth forceps (thus excluding connective tissue elements) and, secondly, considerable axonal growth has been reported through the resulting lesion. Our previous studies have shown that GAP-43 is extensively distributed in the rat spinal cord (see accompanying paper), but here we have used anti-GAP-43 antiserum at a dilution which did not yield any immunostaining in normal cord. However, supranormal levels of GAP-43 were detected in cell bodies and axons around the lesion within four days of compression injury. Double immunostaining with the RT97 monoclonal antibody indicated that a small subpopulation of neurons local to the site of compression were axotomized and expressed GAP-43 and phosphorylated neurofilament epitopes in their cell bodies. Although damage to long axon tracts was extensive, there was no evidence of regeneration in white matter. On the other hand cavities which formed in grey matter provided an environment for axonal elongation. Immunolabelling with markers for astrocytes and endothelial cells was used to evaluate the interaction of elongating axons with endogenous CNS cell types. Sprouting axons, identified by the presence of elevated levels of GAP-43, did not appear to grow in contact with astrocytes but preliminary evidence suggested that newly formed capillaries provided an appropriate substrate.

Regeneration of adult rat CNS axons into peripheral nerve autografts: ultrastructural studies of the early stages of axonal sprouting and regenerative axonal growth

If one end of a segment of peripheral nerve is inserted into the brain or spinal cord, neuronal perikarya in the vicinity of the graft tip can be labelled with retrogradely transported tracers applied to the distal end of the graft several weeks later, showing that CNS axons can regenerate into and along such grafts. We have used transmission EM to examine some of the cellular responses that underlie this regenerative phenomenon, particularly its early stages. Segments of autologous peroneal or tibial nerve were inserted vertically into the thalamus of anaesthetized adult albino rats. The distal end of the graft was left beneath the scalp. Between five days and two months later the animals were killed and the brains prepared for ultrastructural study. Semi-thin and thin sections through the graft and surrounding brain were examined at two levels 6-7 mm apart in all animals: close to the tip of the graft in the thalamus (proximal graft) and at the top of the cerebral cortex (distal graft). In another series of animals with similar grafts, horseradish peroxidase was applied to the distal end of the graft 24-48 h before death. Examination by LM of appropriately processed serial coronal sections of the brains from these animals confirmed that up to several hundred neurons were retrogradely labelled in the thalamus, particularly in the thalamic reticular nucleus. Between five and 14 days after grafting, large numbers of tiny (0.05-0.20 microns diameter) nonmyelinated axonal profiles, considered to be axonal sprouts, were observed by EM within the narrow zone of abnormal thalamic parenchyma bordering the graft. The sprouts were much more numerous (commonly in large fascicles), smoother surfaced, and more rounded than nonmyelinated axons further from the graft or in corresponding areas on the contralateral side of animals with implants or in normal animals. At longer post-graft survival times, the number of such axons in the parenchyma around the graft declined. At five days, some axonal sprouts had entered the junctional zone between the brain and the graft. By eight days there were many sprouts in the junctional zone and some had penetrated the proximal graft to lie between its basal lamina-enclosed columns of Schwann cells, macrophages and myelin debris. Within the brain, sprouts were in contact predominantly with other sprouts but also with all types of glial cell.(ABSTRACT TRUNCATED AT 400 WORDS)