A continuing signal maintains NGF receptor expression in hypoglossal motor neurons after crush injury

Wide range of lineages of cells expressing nerve growth factor mRNA in the nerve lesions of patients with vasculitic neuropathy: An implication of endoneurial macrophage for nerve regeneration

In situ localization of nerve growth factor (NGF) mRNA was examined in the nerve lesions of patients with vasculitic neuropathy. Double labeling of in situ hybridization for NGF mRNA and immunohistochemistry for cell markers showed that NGF mRNA was expressed in a wide range of lineages of cells: Schwann cells, infiltrating macrophages, T cells and perivascular cells. Round-shaped macrophages with early-phase features expressed high levels of NGF mRNA, in contrast to late-phase polymorphic macrophages, which expressed low levels of NGF mRNA. NGF mRNA was also expressed universally in T cells with various cell surface markers. Epineurial macrophages surrounding vasculitic lesions and endoneurial T cells expressed high levels of NGF mRNA in the damaged nerves. Moreover, the extent of endoneurial NGF expression level in macrophages was closely related to the degree of axonal regeneration. These results suggest that NGF is expressed in a wide range of lineages of cells but is differentially expressed spatially in vasculitic nerve lesions, and that the expressed NGF, particularly in macrophages, may play an important role in the nerve regeneration process.

In vivo expression of the intermediate filament peripherin in rat motoneurons: modulation by inhibitory and stimulatory signals

Peripherin is a type III intermediate filament which, in contrast to the neurofilaments, is strongly up-regulated after nerve injury. Although peripherin expression is stimulated in vitro by neurotrophins and cytokines, little is known about its in vivo regulation. In this report, we show that the in vivo down-regulation of peripherin expression to normal levels during regeneration closely correlates with target reconnection in rat facial motoneurons. Prevention of reconnection, by transection and suture, results in the persistence of strong peripherin expression for prolonged periods of up to 11months. This contrasts with the modulation of the p75 low-affinity neurotrophin receptor, whose expression returns to normal even in the absence of reconnection. We further demonstrate that blockade of the axonal transport in non-injured motoneurons increases the expression of peripherin. Blockade of the axonal transport simultaneously to, or after injury of, facial motoneurons does not abolish the axotomy-induced peripherin up-regulation. These data demonstrate that the in vivo expression of peripherin is normally restrained by a distal retrogradely transported inhibitory signal. Thus, peripherin up-regulation results primarily from a lack of supply in this factor. Our results show that stimulatory factors released at the injury site are not required for the initial up-regulation and maintenance of high peripherin expression. However, they appear to enhance this increase during the acute post-lesion phase. Peripherin expression is thus finely tuned by both glial cell-derived stimulatory and distal inhibitory signals that reflect neuron-target interactions.

Regulation of neurotrophin signaling in aging sensory and motoneurons: dissipation of target support?

A hallmark of senescence is sensorimotor impairment, involving locomotion and postural control as well as fine-tuned movements. Sensory and motoneurons are not lost to any significant degree with advancing age, but do show characteristic changes in gene-expression pattern, morphology, and connectivity. This review covers recent experimental findings corroborating that alterations in trophic signaling may induce several of the phenotypic changes seen in primary sensory and motoneurons during aging. Furthermore, the data suggests that target failure, and/or breakdown of neuron-target interaction, is a critical event in the aging process of sensory and motoneurons.

Effects of aging and axotomy on the expression of neurotrophin receptors in primary sensory neurons

Aging is accompanied by declined sensory perception, paralleled by widespread dystrophic and degenerative changes in both central and peripheral sensory pathways. Several lines of evidence indicate that neurotrophic interactions are of importance for a maintained plasticity in the adult and aging nervous system, and that changes in the expression of neurotrophins and/or their receptors may underpin senile neurodegeneration. We have here examined the expression of neurotrophin receptor (p75NTR, trkA, trkB, and trkC) mRNA and protein in intact and axotomized primary sensory neurons of young adult (3 months) and aged (30 months) rats. To examine possible differences among primary sensory neuron populations, we have studied trigeminal ganglia (TG) as well as cervical and lumbar dorsal root ganglia (DRG). In intact aged rats, a decrease in trk (A/B/C) mRNA labeling densities and protein-like immunoreactivities was observed. The decrease was most pronounced in lumbar DRG. In contrast, a small, not statistically significant, increase of p75NTR expression was observed in aged DRG neuron profiles. After axotomy, a down-regulation of mRNA and protein levels was observed for all neurotrophin receptors (p75NTR, trkA, trkB and trkC) in both young adult and aged rats. Consistent with the higher expression levels of neurotrophin receptors in unlesioned young adult primary sensory neurons, the relative effect of axotomy was more pronounced in the young adult than aged rats. Although a decrease in mean cell profile cross-sectional areas was found during aging and after axotomy, the characteristic distribution of neurotrophin receptor expression in different populations of NRG neurons was conserved. The present findings suggest an attenuation of neurotrophic signaling in primary sensory neurons with advancing age and that the expression of p75NTR and trks is regulated differently during aging. A similar dissociation of p75NTR and trk regulation has previously been reported in other neuronal systems during aging, suggesting that there may be a common underlying mechanism. Decreased access to ligands, disturbed axon function and systemic changes in androgen/estrogen levels are discussed as inducing and/or contributing factors.

Nature of the retrograde signal from injured nerves that induces interleukin-6 mRNA in neurons

In previous studies, interleukin-6 was shown to be synthesized in approximately one-third of lumbar dorsal root ganglion neurons during the first week after nerve transection. In present studies, interleukin-6 mRNA was found to be induced also in axotomized facial motor neurons and sympathetic neurons. The nature of the signal that induces interleukin-6 mRNA in neurons after nerve injury was analyzed. Blocking of retrograde axonal transport by injection of colchicine into an otherwise normal nerve did not induce interleukin-6 mRNA in primary sensory neurons, but injection of colchicine into the nerve stump prevented induction of interleukin-6 mRNA by nerve transection. Therefore, it was concluded that interleukin-6 is induced by an injury factor arising from the nerve stump rather than by interruption of normal retrograde trophic support from target tissues or distal nerve segments. Next, injection into the nerve of a mast cell degranulating agent was shown to stimulate interleukin-6 mRNA in sensory neurons and systemic administration of mast cell stabilizing agents to mitigate the induction of interleukin-6 mRNA in sensory neurons after nerve injury. These data implicate mast cells as one possible source of the factors that lead to induction of interleukin-6 mRNA after nerve injury. In search of a possible function of inducible interelukin-6, neuronal death after nerve transection was assessed in mice with null deletion of the interleukin-6 gene. Retrograde death of neurons in the fifth lumbar dorsal root ganglion was 45% greater in knockout than in wild-type mice. Thus, endogenous interleukin-6 contributes to the survival of axotomized neurons.

Re-expression of p75NTR by adult motor neurons after axotomy is triggered by retrograde transport of a positive signal from axons regrowing through damaged or denervated peripheral nerve tissue

To investigate different types of potential signalling mechanisms that regulate neuronal reactions to axotomizing injury, we compared the re-expression of the low-affinity neurotrophin receptor, p75NTR, and the down-regulation of choline acetyltransferase expression, after various combinations of axotomy, crush injury and blockade of axonal transport in adult hypoglossal motor neurons in the rat. We found that pure axotomy in the absence of crush injury down-regulated choline acetyltransferase, but did not induce p75NTR re-expression. Blockade of axonal transport with colchicine had an identical effect. In contrast, both a crush injury on its own, or a crush injury proximal to a complete axotomy, induced p75NTR re-expression and down-regulated expression of choline acetyltransferase. Blockade of axonal transport with colchicine or tight ligation proximal to a crush prevented the crush injury-induced re-expression of p75NTR. Infusion of vehicle, nerve growth factor or ciliary neurotrophic factor induced low levels of p75NTR re-expression that were not significantly different from each other and were substantially lower than crush-induced levels. These findings confirm previous suggestions that the loss of choline acetyltransferase expression is due to the interruption of a constitutive retrograde signal, and show that the re-expression of p75NTR by adult motor neurons after axotomy is triggered by the retrograde transport of a positive signal derived from axons that are regrowing through damaged or denervated peripheral nerve tissue. The precise source and nature of this signal are not yet clear.

Expression of p75(NTR), trkB and trkC in nonmanipulated and axotomized motoneurons of aged rats

Several lines of evidence indicate that adult neurons remain dependent on neurotrophins and that changes in tissue expression of neurotrophins and/or their receptors may play a role in senile neurodegeneration. We have studied the expression of p75NTR, trkB and trkC, respectively, in lumbar motoneurons of young adult (2-3 months) and aged (30 months) rats subjected to sciatic transection using in situ hybridization and immunohistochemistry. Nonmanipulated age-matched animals were processed in parallel. In nonmanipulated aged rats, high levels of p75NTR could be seen in a number of motoneurons (10-15%), while in young adult animals no p75NTR could be detected. Seven days following sciatic axotomy, a conspicuous ipsilateral upregulation p75NTR was observed in young adult rats. Also in aged rats there was a marked ipsilateral increase in number of p75NTR expressing neurons ( approximately 100%). In comparison to young adult rats, aged rats showed a decreased expression of both trkB (5/6 animals) and trkC (6/6 animals). Furthermore, in response to sciatic transection, 3 out of 5 aged rats did not show an increased expression of trkB. In aged rats, axotomy did not induce any significant change in trkC expression. In the young adult rats, we recorded a side-to-side effect with lower values ipsilaterally, however, it cannot be excluded that this difference was caused by an upregulation in the contralateral motoneurons. Oligonucleotide probes against BDNF and NT3 mRNA showed only very few faintly positive neurons in both age groups. Our results indicate that the pattern of regulatory changes of NT receptors in response to axotomy is different in aged and young adult rats. The lack of covariation between p75NTR and trkB and trkC regulation in aged rats indicates a changed role for p75NTR in senescent motoneurons.

Neurotrophin receptor expression is induced in a subpopulation of trigeminal neurons that label by retrograde transport of NGF or fluoro-gold following tooth injury

Tissue responses to injury are regulated by neurotrophins and neurotrophin receptor levels and can involve both retrograde and paracrine/autocrine trophic signaling. To determine how neurotrophins may contribute to the injury response, the timing and the extent of the up-regulation of neurotrophins and their receptors was examined in a model system which is particularly well suited for the analysis of trophic signaling pathways in response to injury. Injury to the occlusal surfaces of rat molar cusps induces a localized increase in nerve growth factor (NGF) expression in the dental pulp within 4-6 h. Radiolabeled NGF was transported in a receptor-mediated fashion from the teeth to a subset of neurons in the trigeminal ganglion within 15 h, indicating that these neurons possess NGF receptors (trk A and/or p75NTR). To test for NGF responses in the tooth sensory afferent neurons, levels of expression of neurotrophins and their receptors were examined by in situ hybridization in the trigeminal ganglion at 0, 4, 12, 20, 28 and 52 h post-injury. Within the maxillary division of the trigeminal ganglion, trk A expression was elevated at 4 h post-injury, with a maximum increase (2-fold) after 52 h. p75NTR was increased by 28 h post-injury and was increased 1.35-fold by 52 h. BDNF mRNA was increased 12 h after injury (1.8-fold), and 2.5-3-fold at 52 h post-injury. The trk B expression was increased only late after injury (28 and 52 h). To determine the receptor/neurotrophin phenotype of trigeminal neurons with projections to the molar teeth, these neurons were double-labeled with the retrograde tracer fluoro-gold and probes for either BDNF or trk B. The results show that tooth-innervating trigeminal neurons express BDNF, but not trk B. The timing of mRNA expression after injury and the phenotype of identified trigeminal neurons suggests a complex signaling cascade in which NGF at the injury site regulates NGF receptor expression at the levels of the cell body as well as increases in BDNF expression. Upregulated BDNF may act in a paracrine fashion on neighboring trigeminal cells expressing trk B. This signaling cascade may be a common feature of the response to mild peripheral inflammatory injuries within nociceptive pathways.

Differential expression of the p75 nerve growth factor receptor in glia and neurons of the rat dorsal root ganglia after peripheral nerve transection

Sympathetic nerve terminals on blood vessels within the dorsal root ganglia sprout after sciatic nerve lesions in the rat. The mechanism underlying this phenomenon is not clear, but might be predicted to involve nerve growth factor or its homologs because these factors are known to trigger collateral sprouting of undamaged sympathetic noradrenergic terminals. We have found that sciatic nerve lesions lead to a decreased expression of neuronal p75, the low-affinity receptor for the neurotrophins, but an increased expression of glial p75 in ipsilateral dorsal root ganglia. Intriguingly, the increased expression of p75 was found primarily in association with glia surrounding large-diameter neurons, which are those associated with the noradrenergic sprouts. A smaller but significant glial response was also found in contralateral ganglia. The glial response in ipsilateral ganglia could be mimicked by ventral, but not dorsal, root transection. The dorsal root lesion-induced glial responses in contralateral ganglia were greater than those induced by ventral root or sciatic nerve lesions. Combined lesions of dorsal root and either ventral root or sciatic nerve did not prevent the glial responses of ipsilateral ganglia, suggesting that a peripheral signal is involved. Colocalization studies indicate that tyrosine hydroxylase-immunoreactive nerve sprouts were associated with p75-immunoreactive glial cells. Thus, increased glial synthesis of p75 might provide an explanation for the abnormal growth of sympathetic fibers in dorsal root ganglia after peripheral nerve injury.

Axotomy induces a different modulation of both low-affinity nerve growth factor receptor and choline acetyltransferase between adult rat spinal and brainstem motoneurons

Adult rat spinal and brainstem motoneurons re-express low-affinity nerve growth factor receptor (p75) after their axotomy. We have previously reported and quantified the time course of this reexpression in spinal motoneurons following several types of injuries of the sciatic nerve. Other studies reported the reexpression of p75 in axotomized brainstem motoneurons. Results of these previous studies differed regarding the type of the most effective triggering injury for p75 reexpression, the relative duration of this reexpression and the decrease of choline acetyltransferase (ChAT) immunoreactivity (-IR) following a permanent axotomy of spinal or brainstem motoneurons. These differences suggest that these two populations of motoneurons respond to axotomy with a different modulation of p75 and ChAT expression. The aim of the present study was to determine whether differential modulation exists. We have analyzed and quantified the presence of p75- and ChAT-IR motoneurons in the hypoglossal nucleus following the same types of injury and the same time course we previously used for sciatic motoneurons. The results show that a nerve crush is the most effective triggering injury for p75 and that it induces similar temporal patterns of p75 and ChAT expression for sciatic and hypoglossal motoneurons. In contrast, a cut injury of the sciatic and hypoglossal nerves resulted in distinct temporal courses of both p75 and ChAT expression between these two populations of motoneurons. In fact, a permanent axotomy of the hypoglossal motoneurons induced i) a much longer maintenance phase for p75 than in sciatic motoneurons and ii) a progressive loss of ChAT-IR with a successive return to normal values in contrast to the modest decrease in the sciatic motoneurons. This evidence indicates that spinal and brainstem motoneurons respond to a permanent axotomy with a different modulation of p75 and ChAT expression. Altogether, the present data and the reported evidence of a differential post-axotomy cell death support the hypothesis that these two populations of motoneurons undergo different dynamic changes after axotomy.

Cell body response to injury in motoneurons and primary sensory neurons of a mutant mouse, Ola (Wld), in which Wallerian degeneration is delayed

We examined the response to axon injury in the facial motoneurons and dorsal root ganglion (DRG) neurons of C57BL/Ola (Wld) mice, compared with the responses of C57BL/6J mice. The peripheral nerves of Ola mutants undergo remarkably slowed and muted Wallerian degeneration after injury. The increase in GAP-43 mRNA levels in facial motoneurons and DRG neurons was similar in both strains of mice, as was the initial decrease in medium-weight neurofilament (NFM) mRNA in facial motoneurons, and the increase in JUN immunoreactivity in both types of neurons. However, the subsequent recovery to normal low levels of JUN and GAP-43 mRNA expression and high levels of NFM mRNA was delayed in Ola motoneurons. We ascribe this delay to the slow regeneration and target reinnervation of facial axons in the Ola mice. These results show that absence of rapid Wallerian degeneration does not affect the initial cell body response to axonal injury. They also provide further evidence that restoration of normal levels of expression of GAP-43 and NFM mRNAs is dependent on target reinnervation and/or trophic factors provided by the distal nerve. Impaired regeneration in the Ola mouse does not seem to be a consequence of a defective cell body response to injury, and our results illustrate the general principle that, even if there is a vigorous cell body response to injury, normal axonal regeneration requires the additional provision of a favorable environment for growth.

Experimental model for local administration of nerve growth factor in microsurgical nerve reconnections

An experimental model for local administration of neuronotrophic substances at the site of peripheral nerve lesion is presented. The model consists of a subcutaneously located silicone reservoir and a connecting tube with its distal end fixed in the proximity of the severed and repaired nerve. The results of the preliminary tests of the model are presented. Sixty Sprague-Dawley rats were divided into two groups: control (saline-treated) (n = 30) and NGF-treated (n = 30). After axotomy of the sciatic nerve, an epineurial repair is performed. NGF or saline is injected daily into the subcutaneous reservoir during the first 3 weeks after axotomy and a single dose in the fourth week. The regenerated nerve observed in the NGF-treated group after four weeks of treatment presents a greater percentage of myelinated axons, thicker myeline sheaths, and more mature endoneurial layers. This experimental model provides a reliable and quantitative way to deliver neuronotrophic substances in site and at different administration rates.

Differential regulation of c-JUN expression in rat retinal ganglion cells after proximal and distal optic nerve transection

In adult rats, the distance between the site of axotomy and the cell body influences the ability of retinal ganglion cells (RGCs) to survive and regenerate an axon. Optic nerves of adult rats were transected 2 mm or 6 mm from the optic bulb and RGCs labeled with the fluorescent marker fluorogold. The time course of c-JUN expression was monitored immunocytochemically between 3 days and 4 weeks after axotomy. c-JUN expression began later and declined faster after distal axotomy than after proximal transection of the optic nerve. However, at the same time significantly more RGCs survived after distal than after proximal axotomy 3 and 4 weeks after axotomy. These findings suggest that survival and expression of c-JUN in rat RGCs are independently regulated by the length of the optic nerve stump after lesion.

Regulation of immediate-early gene expression in rat retinal ganglion cells after axotomy and during regeneration through a peripheral nerve graft

To determine mechanisms of structural plasticity in adult CNS neurons, we investigated the expression of immediate early genes (IEGs) in the rat retina. Gene products of different IEG families (JUN and FOS proteins) and cAMP-responsive element binding protein (CREBP) were examined by immunohistochemistry under three different paradigms. Normal rats which were not axotomized were compared with axotomized animals, were retinal ganglion cells (RGCs) were axotomized by intraorbital optic nerve cut and retrogradely labeled with fluorogold (FG). Under these circumstances, RGCs show only transient sprouting, followed by continuous retrograde RGC degeneration. In the third group, after the optic nerve lesion, adult rats additionally received a sciatic nerve graft to the transected optic nerve stump. This allows some RGCs to regenerate an axon into the grafted nerve. In both groups, the time course of RGC survival and JUN, CREB, and FOS protein expression was monitored. In normal animals, JUN-Immunoreactivity (JUN-Ir) was not detectable in the retinal ganglion cell layer. JUN-Ir was induced in about 70% of all FG-positive RGCs 5 days after axotomy. The expression of JUN-Ir stated to decline 8 days after axotomy. Only a few JUN-Ir-positive RGCs were found after 2 weeks. In transplanted animals, however, the numbers of JUN-Ir-positive RGCs were significantly higher 2 and 3 weeks after transplantation compared to animals that exclusively received axotomy. Furthermore, in grafted rats, about 70% of the regenerating RGCs expressed JUN-Ir 2 weeks after grafting as compared to only 38% JUN-positive RGCs among the surviving but not regenerating RGCs. In normal animals CREBP-Ir was constitutively expressed in nearly all cells of the retinal ganglion cell layer. The decline in number of CREBP-Ir-positive cells paralleled the axotomy-induced RGC death. FOS-Ir-positive cells were not found in the ganglion cell layer at any time. These results demonstrate a selective and transient JUN expression of RGCs after axotomy which is sustained during axonal regeneration. This suggests that sciatic nerve grafts are able to regulate the expression of JUN proteins in axotomized RGCs of adult rats.

Modulation of low-affinity nerve growth factor receptor in injured adult rat spinal cord motoneurons

Spinal and brainstem motoneurons of the adult rat reexpress low-affinity nerve growth factor receptor (LNGFR) and its mRNA after axotomy. We have previously reported the time courses of this reexpression after cut (no regeneration) or crush (followed by regeneration) of the sciatic nerve. We have shown that the length of the different phases of this reexpression (appearance, maintenance and disappearance) can vary according to the type of axotomy. With the present study we expand our previous data and describe and analyze the modulation the LNGFR expression in adult spinal cord motoneurons following different lesion paradigms. In one approach we have imposed three traumatic injuries that still allow regeneration of the sciatic nerve but with a different time course with respect to the crush injury (application of a silicone regeneration chamber, multiple crushes and delayed repair of ligated nerves). In a second approach, we have determined the capability of three toxic or metabolic injuries to induce LNGFR expression without any direct trauma of the nerve (experimental diabetogenesis, botulinum and alpha-bungarotoxin intoxication and 2,5-hexanedione intoxication). In a third approach, we have investigated the effect of the block of the axoplasmic transport on the LNGFR expression following different topical applications of vincristine combined with a nerve crush. The results we present are consistent with the idea that: (1) LNGFR immunoreactivity in adult motoneurons is expressed by motoneurons that are attending to an axonal outgrowth and not a generic signal of cellular damage or impairment of the motor function; (2) LNGFR expression in these motoneurons is related to and parallels the outgrowth process time frame, and (3) the signal/s that trigger and sustain this reexpression may be retrogradely transported from the periphery.

Regulation of aberrant neurofilament phosphorylation in neuronal perikarya. IV. Evidence for the involvement of two signals

Axonal regeneration over long distances is dependent upon events occurring both in the distal stump and in the neuronal cell body. Little is known concerning how events in the distal stump influence the cell body response to injury, or the axon reaction. In the present study, we examined this relationship for one component of the axon reaction (i.e. aberrant neurofilament (NF) phosphorylation) in the C57BL/Ola (Ola) mouse mutant, a model which exhibits delayed Wallerian degeneration (up to 3 weeks) and retarded regeneration of sensory neurons. Non-axotomized normal (C57/6J/BL) and Ola mice demonstrated modest immunostaining to phosphorylated NF (pNF) epitopes (using monoclonal antibody 06-17) in some (11%) L4 dorsal root ganglion (DRG) neuronal cell bodies. In normal mice, modest to intense immunoreactivity was present in 43% of DRG neurons at 1 week following a sciatic nerve crush (axotomy). The intensity and extent of staining declined with reinnervation, being reduced slightly at 2 weeks and more notably by 3 weeks following axotomy. In Ola mice, the intensity and extent (43%) of staining were not different from normal axotomized mice at 1 week following axotomy. However, the intensity was less and the extent of staining reduced by 28% at 2 weeks following axotomy. By 3 weeks, staining levels were again increased, being similar to that observed in Ola and normal mice at 1 week following axotomy.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral nerve regeneration: role of growth factors and their receptors

Growth factors play a central role in the regulation of normal and injury-induced regenerative cell growth. The purpose of this article is to summarize the available data on the expression of different growth factors and their receptors in the injured peripheral nervous system and to discuss their possible role in promoting peripheral nerve regeneration.

Nerve growth factor and regeneration of peripheral nervous system

Nerve growth factor, a well-known neurotrophic factor, supports the survival, differentiation and maintenance of sensory and sympathetic neurons during embryonic development and in the adult. This paper summarises the data on its involvement in peripheral nerve regeneration.