Molecular and cellular aspects of nerve regeneration

La réparation nerveuse périphérique : 30 siècles de recherche

INTRODUCTION

Nerve injury compromises sensory and motor functions. Techniques of peripheral nerve repair are based on our knowledge regarding regeneration. Microsurgical techniques introduced in the late 1950s and widely developed for the past 20 years have improved repairs. However, functional recovery following a peripheral mixed nerve injury is still incomplete.

STATE OF ART

Good motor and sensory function after nerve injury depends on the reinnervation of the motor end plates and sensory receptors. Nerve regeneration does not begin if the cell body has not survived the initial injury or if it is unable to initiate regeneration. The regenerated axons must reach and reinnervate the appropriate target end-organs in a timely fashion. Recovery of motor function requires a critical number of motor axons reinnervating the muscle fibers. Sensory recovery is possible if the delay in reinnervation is short. Many additional factors influence the success of nerve repair or reconstruction. The timing of the repair, the level of injury, the extent of the zone of injury, the technical skill of the surgeon, and the method of repair and reconstruction contribute to the functional outcome after nerve injury.

CONCLUSION

This review presents the recent advances in understanding of neural regeneration and their application to the management of primary repairs and nerve gaps.

Active Src expression is induced after rat peripheral nerve injury

The non-receptor-type Src tyrosine kinases are key components of intracellular signal transduction that are expressed at high levels in the nervous system. To improve understanding of the cascades of molecular events underlying peripheral nerve regeneration, we analyzed active Src expression in the crushed or cut rat sciatic nerves using a monoclonal antibody (clone 28) that recognizes the active form of Src tyrosine kinases, including c-Src and c-Fyn. Western blots showed that active Src expressed in the normal sciatic nerve transiently increased up to threefolds after both types of injury. Immunohistochemistry using clone 28 showed that axonal components are the primary sites of active Src expression in the normal sciatic nerve. Soon after both types of injury, active Src was abundantly expressed in Schwann cells of the segments distal to the injury site. The expression of active Src in the cells decreased with restoration of the axon-Schwann cell relationship and eventually became depleted to very low levels after crushing, but was sustained at high levels in the cut model until the end of the experiment. Regenerated axons consistently expressed active Src throughout nerve regeneration and these eventually became the major sites of active Src expression in the crushed nerve. Among the Src tyrosine kinases, active c-Src selectively increased after crushing according to immunoprecipitation and immunoblotting analyses. Due to its potent biological activity, the increased amounts of the active form of Src probably enhance axonal regrowth, the Schwann cell response, and axon-Schwann cell contact for peripheral nerve regeneration.

Platelet-derived growth factor-b expression induced after rat peripheral nerve injuries

Schwann cells are crucially important for peripheral nerve regeneration. These cells synthesize several factors that are supposed to enhance axonal regeneration when injured. Platelet-derived growth factor (PDGF) B-chain and its beta-receptor are expressed in Schwann cells in both normal peripheral nerves and culture. To elucidate the role of PDGF-B in peripheral nerve regeneration, we investigated its expression in cut or crush-injured rat sciatic nerves for up to 28 days. Northern blotting identified substantial increase of PDGF B-chain transcripts in injured nerves. Immunohistochemistry demonstrated that protein products of the transcripts were augmented at the distal tip of swollen axons in proximal nerve segments and in regenerating axons. Soon after both types of injury, considerable amounts of PDGF-B accumulated in numerous Schwann cells in distal segments of both models. With restoration of the axon-Schwann cell relationship in the crush model, levels of PDGF-B tended to decrease, eventually returning to normal. In the cut model in which the relationship cannot be restored, the PDGF-B was depleted to a very low level. The spatiotemporal correlation between PDGF-B and cell proliferation was very close throughout the study. These results differed strikingly from those of our previous study of rat optic nerve transection, in which PDGF-B was expressed only in a few recruited macrophages and glial cells. Augmented PDGF-B expression after sciatic nerve injury might contribute to peripheral nerve regeneration because PDGF-B is a mitogen and survival factor for Schwann cells and because it has trophic activity on neurons.

Electromagnetic fields influence NGF activity and levels following sciatic nerve transection

Pulsed electromagnetic fields (PEMF) have been shown to increase the rate of nerve regeneration. Transient post-transection loss of target-derived nerve growth factor (NGF) is one mechanism proposed to signal induction of early nerve regenerative events. We tested the hypothesis that PEMF alter levels of NGF activity and protein in injured nerve and/or dorsal root ganglia (DRG) during the first stages of regeneration (6-72 hr). Rats with a transection injury to the midthigh portion of the sciatic nerve on one side were exposed to PEMF or sham control PEMF for 4 hr/day for different time periods. NGF-like activity was determined in DRG, in 5-mm nerve segments proximal and distal to the transection site and in a corresponding 5-mm segment of the contralateral nonoperated nerve. NGF-like activity of coded tissue samples was measured in a blinded fashion using the chick DRG sensory neuron bioassay. Overall, PEMF caused a significant decrease in NGF-like activity in nerve tissue (P < 0.02, repeated measures analysis of variance, ANOVA) with decreases evident in proximal, distal, and contralateral nonoperated nerve. Unexpectedly, transection was also found to cause a significant (P=0.001) 2-fold increase in DRG NGF-like activity between 6 and 24 hr postinjury in contralateral but not ipsilateral DRG. PEMF also reduced NGF-like activity in DRG, although this decrease did not reach statistical significance. Assessment of the same nerve and DRG samples using ELISA and NGF-specific antibodies confirmed an overall significant (P < 0.001) decrease in NGF levels in PEMF-treated nerve tissue, while no decrease was detected in DRG or in nerve samples harvested from PEMF-treated uninjured rats. These findings demonstrate that PEMF can affect growth factor activity and levels, and raise the possibility that PEMF might promote nerve regeneration by amplifying the early postinjury decline in NGF activity.

Differentially expressed genes after peripheral nerve injury

In an attempt to identify genes associated with Wallerian degeneration and peripheral nerve regeneration we have performed differential hybridization screening of a cDNA library from crushed rat sciatic nerve (7 days postlesion) using radioactively labeled cDNA prepared from poly(A)+ RNA of normal vs. crushed nerve. Screening of 5,000 randomly selected colonies yielded 24 distinct clones that were regulated following nerve injury. Fifteen of the differentially expressed sequences could be classified as induced, whereas 9 sequences appeared to be repressed at 1 week postcrush. Sequencing and computer-assisted sequence comparison revealed 3 classes of regulated cDNA clones representing 1) novel gene sequences (8 clones) including 3 transcripts containing a repetitive "brain identifier" (ID) element; 2) identified genes (7 clones) with previously undetected expression in the peripheral nervous system (PNS), such as apolipoprotein D, peripheral myelin protein 22kD (PMP22), SPARC (secreted protein, acidic and rich in cysteine), sulfated glycoprotein SGP-1, apoferritin, decorin, and X16/SRp20; and 3) identified genes (9 clones) with known expression in the PNS including, e.g., the myelin protein P0, gamma-actin, vimentin, alpha-tubulin, chargerin II, and cytochrome c-oxidase subunit I. Northern blot and polymerase chain reaction analyses with RNA from crushed and transected nerve demonstrated that sequences with related function, like the group of myelin genes, cytoskeleton genes, genes involved in RNA processing and translation, in lipid transport or energy metabolism showed closely related temporal patterns of expression during nerve degeneration and regeneration. Finally, we compared the differentially expressed genes identified at 7 days after crush injury (this investigation) with the regulated sequences isolated previously by De Leon et al. (J Neurosci Res 29:437-488, 1991) from a 3 day postcrush sciatic nerve cDNA library.

Increased production of ubiquitin mRNA in motor neurons after axotomy

Ubiquitin targets proteins for attack by certain proteolytic enzymes, but the ubiquitinated cytoplasmic inclusions seen in some chronic neurodegenerative diseases may indicate the occurrence of reparative rather than destructive metabolic events. We have examined the production of ubiquitin in motor neurons of the rat's left hypoglossal nucleus after transection of their axons in circumstances that favour or prevent axonal regeneration. One week after axotomy, in situ hybridization with a radiolabelled cRNA probe revealed a twofold increase in the ubiquitin mRNA content of neurons with regenerating axons (nerve crushed) but not significant change when axonal regeneration had been prevented (nerve transected and ligated). After 2 weeks, ubiquitin mRNA was elevated to about 1.5 times the contralateral control level, regardless of the type of nerve injury, and by 4 weeks there were no longer any differences between the left and right sides. Despite the increased transcription, axotomy was not followed by any change in the quantity of ubiquitin-immunoreactive material in the nuclei or perikarya of hypoglossal neurons as measured by video image analysis of immunohistochemically stained sections. We suggest that ubiquitin is synthesized in neuronal cell bodies and transported into their axons, and that ubiquitin-mediated proteolysis is a metabolic process involved in the elongation of regenerating axons.

Effects of nerve segment supernatants on cultured Schwann cell proliferation and laminin production

Mouse sciatic nerves were transected and 3 hr to 16 days later proximal segments were removed and homogenized. Supernatants of these segments or of normal sciatic nerves were added to Schwann cells maintained in Dulbecco's modified Eagle's medium (DMEM) + 15% fetal calf serum (FCS). After 6 days, Schwann cells were solubilized and the protein content was measured using a Bio-Rad (Melville, NY) protein assay. Samples containing the same amounts of protein were then applied to microtiter plates and the laminin content was determined by enzyme-linked immunosorbent assay (ELISA). Lysates of cultures treated with 24 hr proximal segment supernatants contained significantly higher levels of laminin than those prepared from other intervals, from distal segments, or from control nerves. Increased surface and cytoplasmic anti-laminin immunoreactivity also was found in Schwann cells treated with 24 hr supernatants. To identify the source(s) of this effect, proximal segments removed 24 hr after transection were bisected; supernatants were prepared from each half and tested. Significant increases in laminin production were produced by supernatants from both halves. When supernatants from proximal and distal halves were compared, the latter produced significantly higher laminin levels. Electron microscopic examination of both halves showed that distal halves contained sprouting neurites and growth cones ensheathed by Schwann cells which had a basal lamina and resembled those seen during development and regeneration. Proximal halves appeared normal. Schwann cell proliferation also was compared in supernatant-treated cultures by using a bromodeoxy-uridine (BrdU) ELISA. The 24 hr and 2 day supernatants increased Schwann cell proliferation significantly; 12 hr, 4 day, and 8 day supernatants produced smaller increases. Our observations suggest that axons undergoing early regenerative changes are one of several possible sources of substance(s) in our proximal segment supernatants which increased Schwann cell proliferation and laminin production.

Characterization of novel set of membrane antigens associated with axonal growth. III: Expression in the regenerating goldfish optic nerve and tectum

In the preceding two papers in this series, a polyclonal antiserum (3070) and several monoclonal antibodies (MAbs 4 and 199) were used to define a novel set of laminin-binding membrane antigens in chick brain. In chick brain, 3070 and MAb 4 antigens stained growing axons and migrating nascent neurons transiently; though projection neurons were intensely labelled at all stages, normal glial cells were stained faintly or not at all. To learn if these antigens are associated more generally with active states of nerve growth, the present paper examined the expression of 3070 and MAb 4 antigens during regeneration of the goldfish optic nerve. Only a few optic fibers or glial cells were stained by 3070 antiserum in the resting (uninjured) optic nerve, but 10 days following a unilateral nerve crush, when the growth response is maximal in these fish, distal regenerating neuritic sprouts and glial cell bodies and processes were profusely labelled. The intensity of labelling was diminished by 135 days post-crush, when most active growth was completed. Since 3070/MAb 4 antigens exhibit properties expected for functional laminin receptors, these findings suggest several possible roles that they could play to aid nerve regeneration.

Neurite regeneration from single primary-auditory neurons in vitro

Neurons of the VIIIth cranial nerve in lower vertebrates precisely reconnect with their targets after sustaining injury. It is not known, however, whether the regenerating neurites are guided entirely by external cues or may also be directed by intrinsic mechanisms. To address this issue, single adult primary-auditory neurons were dissected from goldfish and placed in an in vitro environment, devoid of the normal complement of satellite cells, neighboring neurons, and synaptic targets, to observe their patterns of growth. Because acutely isolated neurons showed little neurite outgrowth, neurite regeneration was enhanced by focally crushing the VIIIth cranial nerve 2 to 24 h prior to removal for tissue culture. Neurons that regenerated under identical culture conditions showed growth patterns that could be categorized into three separate groups based on both their morphology and growth patterns. They either 1) remained unbranched (54%), 2) bifurcated or trifurcated into major branches directly from the myelinated stump (V-shaped) (19%), or 3) bifurcated from a regenerated process (Y-shaped), sometimes with a third, smaller branch (27%). Unbranched and V-shaped neurites grew at a constant elongation rate, while Y-shaped neurites grew variably, with alternating retractions and elongations. Neurite elongation was completed in a uniform time period of approximately 15 days despite the differences in elongation rate, maximum length, and latency to growth onset. The neurite branching morphology and manner of growth revealed in this study indicated that adult regenerating neurons can reproduce some elements of the final branching patterns in the absence of extrinsic cues, a capability which may ultimately contribute to the fidelity of reconnection seen in vivo.

Isolation and sequence analysis of two intermediate filament cDNA clones from fish optic nerve

The high post-traumatic regenerative ability of fish central nervous system has been partially attributed to the hospitable nature of the surrounding non-neuronal cells and their appropriate response to injury. Uncovering the correlation between fish non-neuronal cell structure and behavior might yield a better understanding of what makes them supportive to axonal growth. Towards this goal, structural proteins expressed by fish non-neuronal cells need to be characterized. In the present study we isolated cDNA clones encoding fish intermediate filaments which are prominent structural proteins in astrocytes. Among the isolated clones, one was identified as fish vimentin and another was found identical to the cloned fish keratin 8. Results are discussed with respect to the use of these cDNAs for further understanding of fish non-neuronal cell plasticity.

Identification of transcriptionally regulated genes after sciatic nerve injury

Mammalian peripheral nerve fibres can regenerate after injury. In an attempt toward a better understanding of the underlying molecular events, we have isolated novel and known rat cDNA sequences, the expression of which are regulated during sciatic nerve regeneration. For this purpose, cDNA libraries were constructed from either the nerve segment distal to the crush site or the corresponding contralateral uninjured nerve of the same animals. These libraries were screened by differential hybridization and several transcriptionally repressed and induced sequences were isolated. Out of 2,000 cDNA clones screened from the distal library, 11 sequences were found to be induced in the distal nerve segment. This set of induced cDNAs included the rat homolog of vimentin, 28 S and 18 S ribosomal RNA species, and two novel sequences. Of 5,000 screened colonies of the contralateral library, 30 colonies contained sequences that were repressed in the distal segment after nerve crush. They were identified as myelin basic protein, myelin P0, alpha-globin, cytochrome oxidase subunit 1, creatine kinase (muscle type, M) and collagen type I. In addition, five novel sequences were found that were dramatically repressed after sciatic nerve crush. Representative clones were tested by northern blot analysis to study their time course of transcriptional regulation during nerve regeneration. The observed patterns suggest that the regeneration phenomenon shows complex gene regulation in which the nonneuronal cells of the distal segment play an important role. Further characterization of the isolated regulated known and unknown sequences will increase our understanding of the molecular events associated with neuronal regeneration.

Differential regulation of S100 beta and mRNAs coding for S100-like proteins (42A and 42C) during development and after lesion of rat sciatic nerve

The changes in the levels of S100 beta (a protein that stimulates neurite extension and neuronal survival) and 42A and 42C (S100-like proteins whose mRNAs are induced in PC12 cells by nerve growth factor) during development and after rat sciatic nerve lesions were analyzed. S100 beta, 42A, and 42C mRNAs showed differential regulation during development. S100 beta mRNA was present both in sciatic nerve and brain, and increased more than 11-fold during the first 3 wk of nerve postnatal development. 42A and 42C mRNAs were essentially restricted to sciatic nerve, with little found in either embryonic or adult brain. The levels of 42C and 42A mRNAs in sciatic nerve increased 4- and 14-fold, respectively, by postnatal day 23 compared to postnatal day 2. 42A, 42C, and S100 beta mRNAs also showed a differential regulation during sciatic nerve degeneration and regeneration. Axotomized and control sciatic nerves were examined by Northern blots at various times after a crush or cut injury. 42A and 42C mRNA levels increased rapidly in the distal segment of axotomized nerve, remained two- to five-fold higher than controls at day 14 after injury but returned to control levels by 40 days. In contrast, S100 beta mRNA showed a three-fold decrease in the axotomized nerve between days 1 and 3 after injury, and slowly returned towards control levels over the next few weeks. The decrease in S100 beta mRNA was reflected by a corresponding decrease in S100 beta protein levels. The induction of 42A and 42C mRNAs and repression of S100 beta mRNA remained if nerve regeneration was prevented.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandin E2 changes in the retina and optic nerve of an eye with injured optic nerve

Changes in arachidonic acid metabolism were studied in the optic nerve, the chorioretina, and in the vitreous following crush injury to the optic nerve of rats. Crush injury led to: (i) a 3.9-fold increase in optic nerve prostaglandin type E2 in vitro production which peaked on day 5 and was followed by a gradual decline, but was still significantly higher than baseline levels by day 12; (ii) a two-fold increase in the chorioretina prostaglandin type E2 in vitro production which peaked on day 1, and resumed baseline levels by day 3; (iii) a 3.5-fold increase in vitreous prostaglandin type E2 levels on day 1 which remained at 1.5-2 times higher than baseline levels for the rest of the study period (12 days). The findings indicate that the pattern of changes in prostaglandin type E2 production by the optic nerve (consisting mostly of white matter) is different from that described for injured brain tissues. The prolonged accumulation of vitreal prostaglandin type E2 in eyes with damaged optic nerve may lead to undesirable effects on the retina beyond those directly manifested in the retina by altered axonal flow in the injured optic nerve.