Tenascin-C expression during wallerian degeneration in C57BL/Wlds mice: possible implications for axonal regeneration

The neuroimmunology of degeneration and regeneration in the peripheral nervous system

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration (WD). WD and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

Epigenetic induction of the Ink4a/Arf locus prevents Schwann cell overproliferation during nerve regeneration and after tumorigenic challenge

The number of Schwann cells is fitted to axonal length in peripheral nerves. This relationship is lost when tumorigenic stimuli induce uncontrolled Schwann cell proliferation, generating tumours such us neurofibromas and schwannomas. Schwann cells also re-enter the cell cycle following nerve injury during the process of Wallerian degeneration. In both cases proliferation is finally arrested. We show that in neurofibroma, the induction of Jmjd3 (jumonji domain containing 3, histone lysine demethylase) removes trimethyl groups on lysine-27 of histone-H3 and epigenetically activates the Ink4a/Arf-locus, forcing Schwann cells towards replicative senescence. Remarkably, blocking this mechanism allows unrestricted proliferation, inducing malignant transformation of neurofibromas. Interestingly, our data suggest that in injured nerves, Schwann cells epigenetically activate the same locus to switch off proliferation and enter the senescence programme. Indeed, when this pathway is genetically blocked, Schwann cells fail to drop out of the cell cycle and continue to proliferate. We postulate that the Ink4a/Arf-locus is expressed as part of a physiological response that prevents uncontrolled proliferation of the de-differentiated Schwann cell generated during nerve regeneration, a response that is also activated to avoid overproliferation after tumorigenic stimuli in the peripheral nervous system.

Critical role of p38 MAPK for regeneration of the sciatic nerve following crush injury in vivo

BACKGROUND

The physiological function of p38α, which is an isoform of p38 MAPK, has been investigated previously in several studies using pharmacological inhibitors. However, the results regarding whether p38α promotes or inhibits nerve regeneration in vivo have been controversial.

METHODS

We generated novel p38α mutant mice (sem mice) with a point mutation in the region encoding the p38α substrate-docking-site, which serves as a limited loss-of-function model of p38α. In the present study, we utilized sem mice and wild-type littermates (wt mice) to investigate the physiological role of p38α in nerve regeneration following crush injuries.

RESULTS

At four weeks after crush injury, the average axon diameter and the average axon area in sem mice were significantly smaller than those in wt mice. The average myelin sheath thickness in sem mice was reduced compared to wt mice, but no significant difference was observed in the G-ratio between the two groups. The sciatic functional index value demonstrated that functional nerve recovery in sem mice following crush injury was delayed, which is consistent with the histological findings. To investigate the underlying mechanisms of these findings, we examined inflammatory responses of the sciatic nerve by immunohistochemistry and western blotting. At an early phase following crush injury, sem mice showed remarkably lower expression of inflammatory cytokines, such as TNF-α and IL-1β, than wt mice. The expression of Caspase-3 and Tenascin-C were also lower in sem mice. Conversely, at a late phase of the response, sem mice showed considerably higher expression of TNF-α and of IL-1β with lower expression of S-100 than wt mice.

CONCLUSIONS

This is the first study of the physiological role of p38 MAPK in nerve regeneration that does not rely on the use of pharmacological inhibitors. Our results indicate that p38α insufficiency may cause an inflammatory disorder, resulting in a delay of histological and functional nerve recovery following crush injury. We conclude that p38 MAPK has an important physiological role in nerve regeneration and may be important for controlling both initiation of inflammation and recovery from nerve injury.

Critical role of p38 MAPK for regeneration of the sciatic nerve following crush injury in vivo

Background

The physiological function of p38α, which is an isoform of p38 MAPK, has been investigated previously in several studies using pharmacological inhibitors. However, the results regarding whether p38α promotes or inhibits nerve regeneration in vivo have been controversial.

Methods

We generated novel p38α mutant mice (sem mice) with a point mutation in the region encoding the p38α substrate-docking-site, which serves as a limited loss-of-function model of p38α. In the present study, we utilized sem mice and wild-type littermates (wt mice) to investigate the physiological role of p38α in nerve regeneration following crush injuries.

Results

At four weeks after crush injury, the average axon diameter and the average axon area in sem mice were significantly smaller than those in wt mice. The average myelin sheath thickness in sem mice was reduced compared to wt mice, but no significant difference was observed in the G-ratio between the two groups. The sciatic functional index value demonstrated that functional nerve recovery in sem mice following crush injury was delayed, which is consistent with the histological findings. To investigate the underlying mechanisms of these findings, we examined inflammatory responses of the sciatic nerve by immunohistochemistry and western blotting. At an early phase following crush injury, sem mice showed remarkably lower expression of inflammatory cytokines, such as TNF-α and IL-1β, than wt mice. The expression of Caspase-3 and Tenascin-C were also lower in sem mice. Conversely, at a late phase of the response, sem mice showed considerably higher expression of TNF-α and of IL-1β with lower expression of S-100 than wt mice.

Conclusions

This is the first study of the physiological role of p38 MAPK in nerve regeneration that does not rely on the use of pharmacological inhibitors. Our results indicate that p38α insufficiency may cause an inflammatory disorder, resulting in a delay of histological and functional nerve recovery following crush injury. We conclude that p38 MAPK has an important physiological role in nerve regeneration and may be important for controlling both initiation of inflammation and recovery from nerve injury.

Identification of novel cell-adhesion molecules in peripheral nerves using a signal-sequence trap

The development and maintenance of myelinated nerves in the PNS requires constant and reciprocal communication between Schwann cells and their associated axons. However, little is known about the nature of the cell-surface molecules that mediate axon-glial interactions at the onset of myelination and during maintenance of the myelin sheath in the adult. Based on the rationale that such molecules contain a signal sequence in order to be presented on the cell surface, we have employed a eukaryotic-based, signal-sequence-trap approach to identify novel secreted and membrane-bound molecules that are expressed in myelinating and non-myelinating Schwann cells. Using cDNA libraries derived from dbcAMP-stimulated primary Schwann cells and 3-day-old rat sciatic nerve mRNAs, we generated an extensive list of novel molecules expressed in myelinating nerves in the PNS. Many of the identified proteins are cell-adhesion molecules (CAMs) and extracellular matrix (ECM) components, most of which have not been described previously in Schwann cells. In addition, we have identified several signaling receptors, growth and differentiation factors, ecto-enzymes and proteins that are associated with the endoplasmic reticulum and the Golgi network. We further examined the expression of several of the novel molecules in Schwann cells in culture and in rat sciatic nerve by primer-specific, real-time PCR and in situ hybridization. Our results indicate that myelinating Schwann cells express a battery of novel CAMs that might mediate their interactions with the underlying axons.

Transcriptional profiling of the injured sciatic nerve of mice carrying the Wld(S) mutant gene: identification of genes involved in neuroprotection, neuroinflammation, and nerve regeneration

Wallerian degeneration (WD) involves the fragmentation of axonal segments disconnected from their cell bodies, segmentation of the myelin sheath, and removal of debris by Schwann cells and immune cells. The removal and downregulation of myelin-associated inhibitors of axonal regeneration and synthesis of growth factors by these two cell types are critical responses to successful nerve repair. Here, we analyzed the transcriptome of the sciatic nerve of mice carrying the Wallerian degeneration slow (Wld(S)) mutant gene, a gene that confers axonal protection in the distal stump after injury, therefore causing significant delays in WD, neuroinflammation, and axonal regeneration. Of the thousands of genes analyzed by microarray, 719 transcripts were differentially expressed between Wld(S) and wild-type (wt) mice. Notably, the Nmnat1, a transcript contained within the sequence of the Wld(S) gene, was upregulated by five to eightfold in the sciatic nerve of naive Wld(S) mice compared with wt. The injured sciatic nerve of wt could be further distinguished from the one of Wld(S) mice by the preferential upregulation of genes involved in axonal processes and plasticity (Chl1, Epha5, Gadd45b, Jun, Nav2, Nptx1, Nrcam, Ntm, Sema4f), inflammation and immunity (Arg1, Lgals3, Megf10, Panx1), growth factors/cytokines and their receptors (Clcf1, Fgf5, Gdnf, Gfrα1, Il7r, Lif, Ngfr/p75(NTR), Shh), and cell adhesion and extracellular matrix (Adam8, Gpc1, Mmp9, Tnc). These results will help understand how the nervous and immune systems interact to modulate nerve repair, and identify the molecules that drive these responses.

Negative regulation of myelination: relevance for development, injury, and demyelinating disease

Dedifferentiation of myelinating Schwann cells is a key feature of nerve injury and demyelinating neuropathies. We review recent evidence that this dedifferentiation depends on activation of specific intracellular signaling molecules that drive the dedifferentiation program. In particular, we discuss the idea that Schwann cells contain negative transcriptional regulators of myelination that functionally complement positive regulators such as Krox-20, and that myelination is therefore determined by a balance between two opposing transcriptional programs. Negative transcriptional regulators should be expressed prior to myelination, downregulated as myelination starts but reactivated as Schwann cells dedifferentiate following injury. The clearest evidence for a factor that works in this way relates to c-Jun, while other factors may include Notch, Sox-2, Pax-3, Id2, Krox-24, and Egr-3. The role of cell-cell signals such as neuregulin-1 and cytoplasmic signaling pathways such as the extracellular-related kinase (ERK)1/2 pathway in promoting dedifferentiation of myelinating cells is also discussed. We also review evidence that neurotrophin 3 (NT3), purinergic signaling, and nitric oxide synthase are involved in suppressing myelination. The realization that myelination is subject to negative as well as positive controls contributes significantly to the understanding of Schwann cell plasticity. Negative regulators are likely to have a major role during injury, because they promote the transformation of damaged nerves to an environment that fosters neuronal survival and axonal regrowth. In neuropathies, however, activation of these pathways is likely to be harmful because they may be key contributors to demyelination, a situation which would open new routes for clinical intervention.

Immunohistochemical labelling of components of the endoneurial extracellular matrix of intact and rhizotomized dorsal and ventral spinal roots of the rat--a quantitative evaluation using image analysis

The endoneurial extracellular matrix (ECM) molecules are involved in cell signalling during nervous system development and regeneration. Quantitative differences of immunofluorescence labelling for chondroitin sulfate proteoglycan (CSPG), fibronectin (FN), tenascin-C (TN-C), and thrombospondin (TSP) were evaluated in intact rat dorsal and ventral roots and dorsal and ventral roots 2 and 4 weeks after rhizotomy using image analysis. The distal stumps of spinal roots displayed increased immunolabelling for the molecules with higher immunofluorescence in dorsal than in ventral roots up to 2 weeks from transection. Four weeks after rhizotomy, the immunoreactivity for CSPG, TN-C and TSP decreased in dorsal and increased in ventral root stumps, although a higher level of immunofluorescence for FN remained in both dorsal and ventral root stumps 4 weeks after injury in comparison to 2 weeks after injury. We suggest that the amount of some ECM molecules changed differentially 2 and 4 weeks after rhizotomy to create an appropriate environment in the endoneurium for early and later regrowth of sensory and motor axons. The results presented here are the first report of differences between the endoneurial ECM content of damaged afferent and motor nerve fibers. In addition, the immunohistochemical detection of individual ECM molecules indicated that final extrinsic conditions stimulating the regrowth of regenerating axons probably arise from a balance of both growth-promoting and -inhibiting molecules in the endoneurium.

Understanding the biology of compressive neuropathies

Compressive neuropathies are highly prevalent, debilitating conditions with variable functional recovery after surgical decompression. Chronic nerve compression injury induces concurrent Schwann cell proliferation and apoptosis in the early stages of the disorder, independent of axonal injury. These proliferating Schwann cells locally demyelinate and remyelinate in the region of injury. Furthermore, Schwann cells upregulate vascular endothelial growth factor secondary to chronic nerve compression injury and induce neovascularization to facilitate the recruitment of macrophages. In contrast to Wallerian degeneration, macrophage recruitment occurs gradually with chronic nerve compression injury and continues for a longer time. Schwann cells change their gene and protein expression in response to mechanical stimuli as shear stress decreases the expression of myelin associated glycoprotein and myelin basic protein mRNA and protein for in vitro promyelinating Schwann cells. The local down-regulation of myelin associated glycoprotein in the region of compression injury creates an environment allowing axonal sprouting that may be reversed with intraneural injections of purified myelin associated glycoprotein. These studies suggest that while the reciprocal relationship between neurons and glial cells is maintained, chronic nerve compression injury is a Schwann cell-mediated disease.

Expression of tenascin R and J1 mRNA in motoneurons after a traumatic lesion in the spinal cord

We demonstrate, using in situ hybridization, that mRNA for the anti-adhesive molecules tenascin R and J1 in the adult rat spinal motoneurons are down-regulated rapidly as a reaction after a ventral funiculus lesion. Tenascin-R was significantly down-regulated at day 1 and normalized after 3 weeks. Tenascin-J1 declined to its lowest value at day 3 and returned to the initial level after 3 weeks. In adjacent sections, the distribution of macrophages was studied with immuno histochemistry. The density of macrophages reached a maximum 3 days after the injury. Thus, the density of macrophages appeared to be inversely related to the level of tenascin mRNA. These data are compatible with the notion that neuronal tenascins may modulate the adhesion of perincurial inflammatory cells.

Compartmental neurodegeneration and synaptic plasticity in the Wld(s) mutant mouse

This review focuses on recent developments in our understanding of neurodegeneration at the mammalian neuromuscular junction. We provide evidence to support a hypothesis of compartmental neurodegeneration, whereby synaptic degeneration occurs by a separate, distinct mechanism from cell body and axonal degeneration. Studies of the spontaneous mutant Wld(s) mouse, in which Wallerian degeneration is characteristically slow, provide key evidence in support of this hypothesis. Some features of synaptic degeneration in the absence of Wallerian degeneration resemble synapse elimination in neonatal muscle. This and other forms of synaptic plasticity may be accessible to further investigations, exploiting advantages afforded by the Wld(s) mutant, or transgenic mice that express the Wld(s) gene.

Differential expression of tenascin-C, tenascin-R, tenascin/J1, and tenascin-X in spinal cord scar tissue and in the olfactory system

The members of the tenascin family are involved in a number of developmental processes, mainly by their ability to regulate cell adhesion. We have here studied the distribution of mRNAs for tenascin-X, -C, and -R and the closely related molecule tenascin/J1 in the olfactory system and spinal cord. The olfactory bulb and nasal mucosa were studied during late embryonic and early postnatal development as well as in the adult. The spinal cord was studied during late embryonic development and after mechanical lesions. In the normal rat, the spinal cord and olfactory bulb displayed similar patterns of tenascin expression. Tenascin-C, tenascin-R, and tenascin/J1 were all expressed in the olfactory bulb and spinal cord during development, while tenascin/J1 was the only extensively expressed tenascin molecule in the adult. In both regions tenascin/J1 was expressed in both nonneuronal and neuronal cells. After a spinal cord lesion, mRNAs for tenascin-C, -X, -R, and/J1 were all upregulated and had their own specific spatial and temporal expression patterns. Thus, even if axonal outgrowth occurs to some extent both in the adult rat primary olfactory system and in spinal cord scar tissue after lesion, the tenascin expression patterns in these two situations are totally different.

Growth promoting and inhibitory effects of glial cells in the mammalian nervous system

In the central nervous system (CNS) of mammals axonal regeneration is limited by two main factors: first, the low intrinsic regenerative potential of adult CNS neurons and second, inhibitory influences of the glial and extracellular environment. Myelin-associated inhibitors of neurite growth as well as some properties of so called "reactive astrocytes" contribute to the non-permissive of CNS tissue for axonal growth. In contrast, the peripheral nervous system (PNS) environment is supportive of regeneration because Schwann cells provide suitable substrates for regrowing axons. Purified PNS myelin, however, inhibits growth of PNS and CNS axons to a similar extent as does CNS myelin. The molecular basis of glial substrate properties has been studied intensively in the recent years and a large number of molecules have been recognized which might play a role in the regulation of axonal growth. Although the exact mechanisms are still not fully understood, accumulating data shed light on the complex interactions between neurons and glia that are required to establish, maintain, and regenerate axonal connections in the nervous system. In the following chapter we review the role of glial cells in the CNS and PNS during processes of de- and regeneration with respect to our own work.

A role of migratory Schwann cells in a conditioning effect of peripheral nerve regeneration

The common peroneal nerve in mice was conditioned by axotomy around the head of the fibula. At various intervals from 1 day to 2, 3, 5, 15, and 25 days, a test lesion was made by axotomy 15 mm proximal to the conditioning lesion site. The proximal stump of the transected nerve was sandwiched between two sheets of thin plastic film and remained in vivo for various intervals from 3 h to 6, 9, 12, 24, 48, 72 and 96 h. The regenerating axons were visualized on the film with silver nitrate impregnation. Schwann cells were visualized migrating onto the film using immunohistochemistry with anti-S-100. To determine the effects of migratory Schwann cells on axonal outgrowth, a film model was established on one limb. After the nerve stump was removed from the film, the treated film was transferred to a new lesion on the contralateral limb and 2 days later the film was harvested for histological examination. Conditioned by a prior axotomy more than 3 days earlier, regenerating axons sprouted within less than 1 h after the test lesion was established and grew naked at five times higher rate: The growth rate was similar to that observed during regeneration in the presence of migratory Schwann cells (ordinary type). After a short interval, the axons, which had been ensheathed by migratory Schwann cells (reactive type), continued growing at a significantly (P < 0.01) higher rate. The reactive type of cells had fewer numbers of branches and higher activity in promoting axonal outgrowth than the ordinary type. Thus, both ordinary and reactive types of cells played key roles in initiating and maintaining a conditioning effect, respectively.

Integrin family of cell adhesion molecules in the injured brain: regulation and cellular localization in the normal and regenerating mouse facial motor nucleus

Integrins are a large family of heterodimeric glycoproteins that play a crucial role in cell adhesion during development, inflammation, and tissue repair. In the current study, we investigated the localization of different integrin subunits in the mouse facial motor nucleus and their regulation after transection of the facial nerve. In the normal mouse brain, there was clear immunoreactivity for alpha5-, alpha6-, and beta1-integrin subunits on blood vessel endothelia and for alphaM- and beta2-subunits on resting parenchymal microglia. Facial nerve transection led to an up-regulation of the beta1-subunit on the axotomized neurons and an increase in the alpha4-, alpha5-, alpha6-, beta1-, alphaM-, alphaX-, and beta2-subunits on the adjacent, activated microglia. Quantification of the microglial integrins revealed two different expression patterns. The subunits alpha5 and alpha6 showed a monophasic increase with a maximum at day 4, the alphaM-subunit a biphasic regulation, with an early peak at day 1 and an elevated plateau between day 14 and 42. At day 14, there was also an influx of lymphocytes immunoreactive for the alpha4beta1- and alphaLbeta2-integrins, which aggregated at sites of neural debris and phagocytotic microglia. This finding was accompanied by a significant increase of the alpha5beta1-integrin on blood vessel endothelia. In summary, facial axotomy is followed by a strong and cell-type-specific expression of integrins on the affected neurons and on surrounding microglia, lymphocytes, and vascular endothelia. The presence of several, strikingly different temporal patterns suggests a selective involvement of these molecules in the different adhesive events during regeneration in the central nervous system.

Loss of distal axons and sensory Merkel cells and features indicative of muscle denervation in hindlimbs of P0-deficient mice

Mice lacking the major Schwann cell myelin component P0 show a severe dysmyelination with pathological features reminiscent of the Déjérine-Sottas syndrome in humans. Previous morphological and electrophysiological studies on these mice did not only demonstrate a compromised myelination and myelin maintenance, but were suggestive of an impairment of axons as well. Here, we studied the axonal pathology in P0-deficient mice by quantitative electron microscopy. In addition, we investigated epidermal receptor end organs by immunocytochemistry and muscle pathology by histochemistry. In proximal sections of facial and femoral nerves, axon calibers were significantly reduced, whereas the number of myelin-competent axons was not diminished in 5- and 17-month-old P0-deficient mice. However, in distal branches of the femoral and sciatic nerve (digital nerves innervating the skin of the first toe) the numbers of myelin-competent axons were reduced by 70% in 6-month-old P0-deficient mice. Immunolabeling of foot pads revealed a corresponding loss of Merkel cells by 75%, suggesting that survival of these cells is dependent on the presence or maintenance of their innervating myelinated axons. In addition, quadriceps and gastrocnemius muscles showed pathological features indicative of denervation and axonal sprouting. These findings demonstrate that loss of an important myelin component can initiate degenerative mechanisms not only in the Schwann cell but also in the distal portions of myelinated axons, leading to the degeneration of specialized receptor end organs and impairment of muscle innervation.

I-type lectins in the nervous system

The number of animal lectins, basically defined upon their interaction with specific carbohydrate structures, is growing considerably during the last few years. Among these proteins the recently identified subfamily of I-type lectins consists of mainly transmembranous glycoproteins belonging to the immunoglobulin superfamily. Most of the I-type lectins participate in cell adhesion events, as are the different sialoadhesins recognizing sialylated glycan structures, which represent the best characterized subgroup. I-type lectins are abundant in the nervous system and have been implicated in a number of morphogenetic processes as fundamental as axon growth, myelin formation and growth factor signaling. In the present review, we summarize the structural and functional properties of I-type lectins expressed in neural tissues with a main focus on the sialoadhesin myelin-associated glycoprotein, the neural cell adhesion molecule and the fibroblast growth factor receptors.

Immunocytochemical characterization of reactive optic nerve astrocytes and meningeal cells

Regeneration in the adult central nervous system (CNS) is thought to be hampered by the lesion-induced activation of astrocytes and meningeal cells and the consecutive formation of a glial scar. The substrate properties of reactive astrocytes differ significantly from their neonatal counterparts, which promote axon growth, but in spite of intensive studies the underlying molecular changes are still not fully understood. We have used two cell culture systems to compare the expression of certain surface molecules on neonatal astrocytes, reactive astrocytes and meningeal cells in vitro. Both, neonatal and reactive adult astrocytes exhibited a very similar expression of growth promoting molecules (NCAM, L1, laminin, fibronectin, DSD-1 proteoglycan) and potential inhibitors (tenascinC, chondroitin sulfate, and NG2-proteoglycan), whereas we could not detect the inhibitory keratan sulfate on either astrocyte population. In contrast, meningeal cells expressed considerable levels of keratan sulfate, but only minimal amounts of NCAM. In addition, the much higher expression of extracellular fibronectin around meningeal cells implies an excess formation of extracellular matrix (ECM). In coculture experiments, embryonic retinal ganglion cell (RGC) axons clearly avoided meningeal cells and instead preferred even reactive adult astrocytes. Our results suggest that the expression of inhibitory keratan sulfate proteoglycans together with a lack of NCAM and an excess production of ECM may be responsible for the non-permissiveness of meningeal cells. Compared to reactive astrocytes, meningeal cells are even worse a substrate for growing axons. None of the molecules investigated, however, seems to account for the different substrate properties of neonatal and reactive adult astrocytes.

Selective inhibition of early axonal regeneration by myelin-associated glycoprotein

When the distal stump of a transected peripheral nerve is brought into the vicinity of the proximal nerve stump, the regenerating axons advance toward it across the gap. Similar results are obtained when a predegenerated nerve segment is used. However, when a nerve segment subjected to proximal axotomy 7 days earlier (7-day nerve segment) was placed close to the proximal end of a freshly cut nerve at a distance of less than 1.5 mm, there were neither regenerating axons nor sprouts. The same inhibition of axonal regeneration was also exhibited when a nerve segment subjected to axotomy 9 to 14 days earlier was used. To examine the inhibitory effect of the nerve segments on established regenerating axons, we positioned a 7-day nerve segment in close apposition to a proximal nerve end at 2 or 3 days after transection. The growth of the 3-day-old regenerating axons, already ensheathed by Schwann cells, was not disturbed, but the 2-day-old regenerating axons, consisting of naked axons, were eliminated by the 7-day nerve segment. It is assumed that the findings reflect a mechanism serving to eliminate abundant sprouts and immature axons, probably conferring optimum regeneration and maturation of outgrowing pioneer axons. The inhibitory effect on abundant sprouts and immature axons was completely blocked by local application of antibodies to myelin-associated glycoprotein (MAG). The MAG-containing cells appeared at 6 to 12 days after axotomy.

Painful mononeuropathy in C57BL/Wld mice with delayed wallerian degeneration: differential effects of cytokine production and nerve regeneration on thermal and mechanical hypersensitivity

Wallerian degeneration with macrophage influx and production of proinflammatory cytokines is a critical factor in the development of hyperalgesia in animal models of neuropathic pain. We hypothesized that in the mouse strain with delayed Wallerian degeneration, the C57BL/Wld mouse, the temporal course of mechanical allodynia and thermal hyperalgesia as well as the temporal profile of cytokine expression after nerve injury would differ from normal mice. Here we used the model of chronic constriction injury of the sciatic nerve (CCI) to study the correlation of pain related behavior with peripheral nerve de- and regeneration and concomitant cytokine production. Indeed, after CCI, C57BL/Wld mice showed markedly reduced thermal hyperalgesia compared to normal C57BL/6 mice, temporally related to the delayed recruitment of hematogeneous macrophages to the injured nerve. Endoneurial tumor necrosis factor-alpha (TNF)-like immunoreactivity increased rapidly in normal mice but did so with a delayed time course in C57BL/Wld mice. In addition, the duration of mechanical allodynia was significantly prolonged in C57BL/Wld mice as compared to C57BL/6 mice, in accordance with the delay in regeneration of sensory nerve fibers in these mice. These results suggest that macrophage invasion and production of TNF may influence the development of thermal hyperalgesia and that regenerative activity is linked to mechanical allodynia in peripheral mononeuropathy.

Autonomic dysfunction in the Lambert-Eaton myasthenic syndrome: serologic and clinical correlates

Autonomic dysfunction is a recognized feature of the Lambert-Eaton myasthenic syndrome (LES). However, the characteristic pattern of dysautonomia has not been clearly documented and its pathophysiologic basis is not known. We therefore abstracted autonomic symptomatology and results of quantitative tests for salivation, and vasomotor, cardiovagal, and sudomotor reflexes from records of 30 LES patients. Dry mouth (77%) and impotence (45% of men) were the most common symptoms. Composite Autonomic Scoring Scale results were abnormal in 93% of patients, and autonomic failure was severe in 20%. The frequency of specific test abnormalities were the following: sudomotor function, 83%; cardiovagal reflexes, 75%; salivation, 44%; and adrenergic function, 37%. Although voltage-gated N-type calcium (Ca2+) channels are implicated in autonomic transmission, the low frequency of serum antibodies to N-type Ca2+ channels found in the patients of this study (31% positive) argues against a pathogenic role in mediating LES-related dysautonomia. In contrast, 93% of the patients were seropositive for P/Q-type Ca2+ channel antibodies. A subset of these antibodies is thought to impair neuromuscular transmission. Autoantibodies of thyrogastric or glutamic acid decarboxylase specificity (markers of predisposition to type 1 diabetes mellitus) were found in 45% of patients, and type 1 antineuronal nuclear antibody (or anti-Hu, a marker of autoimmune neuropathy associated with small-cell lung carcinoma) was found in 3%. No autoantibody correlated with autonomic dysfunction severity. Sensorimotor neuropathy was documented in five patients, and was not significantly associated with autonomic neuropathy. Autonomic failure was most severe in older subjects with cancer (p = 0.02, age by cancer interaction).

The cellular and molecular basis of peripheral nerve regeneration

Functional recovery from peripheral nerve injury and repair depends on a multitude of factors, both intrinsic and extrinsic to neurons. Neuronal survival after axotomy is a prerequisite for regeneration and is facilitated by an array of trophic factors from multiple sources, including neurotrophins, neuropoietic cytokines, insulin-like growth factors (IGFs), and glial-cell-line-derived neurotrophic factors (GDNFs). Axotomized neurons must switch from a transmitting mode to a growth mode and express growth-associated proteins, such as GAP-43, tubulin, and actin, as well as an array of novel neuropeptides and cytokines, all of which have the potential to promote axonal regeneration. Axonal sprouts must reach the distal nerve stump at a time when its growth support is optimal. Schwann cells in the distal stump undergo proliferation and phenotypical changes to prepare the local environment to be favorable for axonal regeneration. Schwann cells play an indispensable role in promoting regeneration by increasing their synthesis of surface cell adhesion molecules (CAMs), such as N-CAM, Ng-CAM/L1, N-cadherin, and L2/HNK-1, by elaborating basement membrane that contains many extracellular matrix proteins, such as laminin, fibronectin, and tenascin, and by producing many neurotrophic factors and their receptors. However, the growth support provided by the distal nerve stump and the capacity of the axotomized neurons to regenerate axons may not be sustained indefinitely. Axonal regenerations may be facilitated by new strategies that enhance the growth potential of neurons and optimize the growth support of the distal nerve stump in combination with prompt nerve repair.

Disruption of the gene for the myelin-associated glycoprotein improves axonal regrowth along myelin in C57BL/Wlds mice

The myelin-associated glycoprotein (MAG) has been shown to be inhibitory for certain neurons in vitro (Mukhopadhyay et al., 1994; McKerracher et al., 1994). To investigate whether MAG is an inhibitory component in peripheral myelin in vivo, MAG-deficient mutant mice were cross-bred with C57BL/Wlds mice that have delayed lesion-induced myelin degeneration and axon regrowth. While in crushed nerves of C57BL/Wlds mice expressing MAG, only 16% of myelin sheaths were associated with regrowing axons, this number was doubled in MAG-deficient C57BL/Wlds mice. These observations suggest that the absence of MAG may contribute to the improved axonal regrowth in the double mutants. Therefore, degeneration of MAG-containing myelin might be an important prerequisite to optimize axonal regrowth after peripheral nerve injury.

Kainic acid activates transient expression of tenascin-C in the adult rat hippocampus

Kainic acid-induced limbic seizures enhance expression of tenascin-C (TN) in the hippocampus of adult rats. TN mRNA was detectable by in situ hybridization in many granule cells in the dentate gyrus 4.5 hr after kainic acid injection but not in saline-injected animals (controls) or in animals killed 2 or 24 hr after injection. Thirty days after kainic acid injection, TN mRNA was detectable only in pyramidal cells of CA3 and CA1. At the protein level, TN was detectable by immunocytochemistry in control animals in the strata oriens and lacunosum moleculare of CA1, in the molecular layer, and within a narrow area at the inner surface of the granule cell layer in the dentate gyrus. Twenty-four hours after kainic acid injection, TN immunoreactivity was enhanced in these areas and throughout the granule cell layer. Thirty days after kainic acid injection, TN immunoreactivity was downregulated in these areas, while it was prominent in the stratum oriens and in clusters of immunoreactivity in the stratum lucidum of CA3. Western blot analysis of the hippocampus showed a peak of TN expression 24 hr after kainic acid injection. These observations show that TN expression is upregulated in predominantly neuronal cells already by 4.5 hr after kainic acid injection, coincident with activation of granule cells and sprouting of axon terminals, whereas the remaining TN expression 30 days after injection relates to pyramidal cells in CA1 and CA3, coincident with an astroglial response, as marked by a strong expression of glial fibrillary acidic protein.

Protein zero (P0)-deficient mice show myelin degeneration in peripheral nerves characteristic of inherited human neuropathies

Mutations in the human gene for the myelin recognition molecule protein zero (P0) give rise to severe and progressive forms of dominantly inherited peripheral neuropathies. We have previously reported that mice homozygous for a null mutation in P0 have severely hypomyelinated nerves ten weeks after birth. Here we show hypomyelination already exists at day four with subsequent demyelination and impaired nerve conduction. Furthermore, heterozygous mutants show normal myelination, but develop progressive demyelination after four months of age. Thus, the pathology of homo- and heterozygous P0 mutants resembles that of the severely affected Déjérine-Sottas and the more mildly affected Charcot-Marie-Tooth type 1B patients, respectively.

Tenascin-C expression by neurons and glial cells in the rat spinal cord: changes during postnatal development and after dorsal root or sciatic nerve injury

We have used in situ hybridization with a digoxigenin-labelled probe for tenascin-C mRNA and immunocytochemistry with antibodies against tenascin-C, glial fibrillary acidic protein, OX-42 and the 200 kDa neurofilament protein to study the expression, distribution and cellular relationships of tenascin-C mRNA and protein in the developing (postnatal) and adult spinal cord of rat, and the effects thereon of dorsal root, ventral root and sciatic nerve injuries. The most interesting finding was that on postnatal day 7 (P7), P14 and in the adult, but not on P0 or P3, a group of neurons in the lumbar ventral horn expressed the tenascin-C mRNA gene. They represented about 5% of ventral horn neurons in the adult and were among the smaller such neurons. Since 40-60% of such cells were lost at P13 following sciatic nerve crush on P0, some were almost certainly motor neurons. In addition, we found that at P0 and P3, mRNA-containing glial cells were widespread in grey and white matter but sparse in the developing dorsal columns; tenascin-C immunofluorescence showed a similar distribution. By P7 there were fewer mRNA-containing cells in the ventral horns and in the area of the dorsal columns containing the developing corticospinal tract where immunofluorescence was also weak. At P14 there were no glial-like mRNA-containing cells in the grey matter; such cells were confined to the periphery of the lateral and ventral white columns but were present throughout the dorsal columns where tenascin-C immunofluorescence was also strong. No glial-like mRNA-containing cells were present in the adult lumbar spinal cord and tenascin-C immunofluorescence was confirmed to irregular patches in the ventral horn, especially around immunonegative cell bodies of small neurons, a zone around the central canal, and a thin zone adjacent to the glia limitans. Thus the expression of tenascin-C is differentially developmentally regulated in the grey matter and in different parts of the white matter. Three days after injury of dorsal roots L4-6, many cells containing tenascin-C mRNA, some identified as glial fibrillary acidic protein-positive astrocytes, were present in the ipsilateral dorsal column, but were rare after longer survivals. Immunoreactivity, however, was elevated in the ipsilateral dorsal column at 3 days, remained high for several months and disappeared at 6.5 months. Dorsal root injury had no effect on tenascin-C mRNA or protein in the grey matter. Sciatic nerve or ventral root injury had no effect on these molecules in any part of the spinal cord.

Crucial role for the myelin-associated glycoprotein in the maintenance of axon-myelin integrity

It has recently been shown that mice deficient in the gene for myelin-associated glycoprotein develop normal myelin sheaths in the peripheral nervous system. Here we report that in mutant mice older than 8 months the maintenance of axon-myelin units is disturbed, resulting in both axon and myelin degeneration. Morphological features include those typically seen in human peripheral neuropathies, where demyelination-induced Schwann cell proliferation and remyelination lead to the formation of so-called onion bulbs. Expression of tenascin-C, a molecule indicative of peripheral nerve degeneration, was up-regulated by axon-deprived Schwann cells and regenerating axons were occasionally seen. Myelin-associated glycoprotein thus appears to play a crucial role in the long-term maintenance of the integrity of both myelin and axons.